Before I get to the "how it is done" part, here are some notes about my patch/hack/fix. Right from the start the major goals have been:
I was able to keep all those goals. Almost everything is done by creating symlinks. Only a single existing symlink had to be replaced and we'll back it up before replacement, just in case.
If you are no expert on terminal operations, I strongly advise you to copy/paste all terminal commands from my reply to your terminal, to avoid typos. Bear in mind that even spacing, quoting and especially capitalization can be important. Copy/paste them line by line, never more than one line at once and hit return after each pasted line to execute the command. Should any operation ever prompt you for a password, this will be the password of the currently logged in administrator user (your keystrokes are not displayed while typing, this is normal, don't worry, just keep typing the password and hit return; re-try if you had a typo and get prompted again).
Before we can start, make sure the following conditions are true:
You are logged in as an administrator user.
You have started Terminal.app (Applications/Utilities) and a terminal window is open.
You have a copy of the Xcode3 (e.g. 3.2.5) and Xcode4 disk image (DMG) or installer available.
If you already have either Xcode version installed, consider uninstalling it first, so you can start with a fresh/clean setup. Uninstalling Xcode will not remove your preferences, color scheme or key binding customizations. Ideally you'd start with a system that has no Xcode version (neither 3 nor 4) currently installed.
Important: Do not install "System Tools" or "Unix Development" package of Xcode3.
Whether you want to install "Mac OS X 10.4 SDK" and/or "Documentation" is up to you. If that is a Xcode3 with iOS SDKs, whether you install those or not is also up to you.
You are free to choose any destination folder for your installation. For this guide I have chosen "/Xcode3", but feel free to pick a different one. Just make sure to alter all terminal commands accordingly.
The order of the steps given here is usually not really important, but I strongly advise you to not swap step 1 and step 2. Xcode always installs a couple of files outside of the chosen destination folder and trust me, in the end you want the Xcode4 versions of those files on your disk. By installing Xcode3 before Xcode4, you can be sure that Xcode4 will overwrite those files if necessary. I once swapped steps 1 and 2 and in the end I had some rather strange issues that might have been related to the incorrect order (I cannot say for sure, but after re-installing in the correct order the issues were gone).
Chose any packets you like. Installing "System Tools" is advisable, but not strictly necessary (though most people will sooner or later miss that functionality).
Again, feel free to pick any target folder you like. For this guide I chose the normal target folder "/Developer", if you take a different one, alter all terminal commands accordingly.
Switch to your terminal window and run the following commands:
Of course only run the command for 10.4u if you also installed SDK 10.4 in step 1.
This is enough to bring the SDKs 10.5 (and possibly 10.4) back to the selection list in Xcode4. Give it a try if you like. Fire up Xcode4, open a project, try changing the selected SDK. That was easy, huh? Be sure to close Xcode4 again (the application, not just the window) before proceeding with the next step.
If you have not installed MacOS 10.4 SDK or if you don't plan to ever use it, you can safely skip this step and proceed with step 5.
To use SDK 10.4, you'll have to use GCC 4.0, GCC 4.2 won't work. Apple claims that GCC 4.2 is not compatible with SDK 10.4, well, if you ask me, this is a hoax. I have already overwritten this limitations more than once and there was never the tiniest issue because of it. It would be easy to modify SDK 10.4 so that Xcode will allow you to use GCC 4.2 for it, but my goal was to avoid all file modifications, so we just add GCC 4.0 support back to Xcode, which is also a good thing, because some projects really depend on GCC 4.0 (e.g. there are some bugs in GCC 4.2 that prevent valid inline assembly code to compile without errors, while the same code compiles flawlessly on GCC 4.0 and GCC 4.4).
Right now we have restored full GCC 4.0 support except for the fact that GCC 4.0 is still not selectable in Xcode4. That is because Xcode4 has no GCC 4.0 compiler plug-in any longer. Fortunately the Xcode3 plug-in also works in Xcode4, only the position has radically changed. Apple now hides those plug-ins deep within a bundle and only plug-ins there seem to work, placing them to their old position seems to have no effect.
Now fire up Xcode4 again, open a project and try selecting the compiler. You should have GCC 4.0 back on the list. Now you can actually already select SDK 10.4 or 10.5, GCC 4.0 and you should have no issue to build a PPC binary. Just select "Other..." for the "Architecture" build setting and manually enter "ppc", then alter "Valid Architectures" to also include "ppc". We are almost done, except that trying to build a PPC binary using GCC 4.2 and SDK 10.5 will still fail.
Since Apple is only supporting Intel platforms in Xcode4, not all GCC 4.2 tools have been built with PPC support. There is one important tool that has no PPC support, the tool is named "as" and it is the GNU Assembler. To compile ppc/ppc64 binaries with GCC 4.2 we need to use an "as" version with ppc/ppc64 support. This is the one and only file (actually it also a symlink) we have to first move aside (making a backup copy) before we can replace it by a symlink:
That's all folks. Considering how easy that was, you can imagine that Apple has certainly not dropped SDK 10.4/10.5 or ppc/ppc64 or GCC 4.0 support because this was a necessity, they dropped all that because they wanted to drop it.
I hope this setup works as well for you as it does for me. I have been able to compile all my old projects in Xcode4 without any major changes, except for having to alter a search path here and there.
It may look strange that I answer my own question here, but since I have found out how to solve this problem all by myself, I'd like to share my knowledge with the community, because I believe this is really valuable input to all MacOS developers out there. This question has been asked so many times in so many places and so far I have never seen anyone coming up with a similar fix. Share the wealth, spread the knowledge and so on, you know what I mean.
If you have additional questions regarding this topic or if you still have problems to build your old projects correctly, please do what Stack Overflow has been designed for: Click on "Ask Question" in the upper right corner of this page and create a new question. That way the whole community can help you solving those issues, since the issues may not directly (maybe not even indirectly) be related to this hack.
I would recommend you mention the fact that you did apply this hack at the very beginning of your question. Maybe you even want to directly link to this question, so that people, who never heard of this hack, can easily look it up. Otherwise most people will get rather confused when you mention SDK 10.4/10.5, PPC or GCC 4.0 in combination with Xcode4, which officially supports neither of these. You might get rather stupid comments instead of decent replies if you forget to mention this hack.
Please refrain from posting your questions or issues here either as replies or as comments. Posting them as replies makes no sense because they are no replies and there is no way how people can reply back to you, other then using comments, and comments may not offer enough room for a decent reply to your question or a decent solution to your problem. And posting them as comments means you are limited to very little room and tracking reply comments will be hard as comments have no tree-like hierarchy (further they will still offer to little room for decent replies/solutions). Thank you.
Of course other kind of comments as well as better replies to the original question are always welcome ;-)
Here's an upvote. I'll probably copy the old SDKs and compiler files instead of just symlinking them in order to be able to uninstall rest of Xcode3. Not because I love Xcode4; solely because I need the disk space. Thanks!
for SRC_FILE in /Xcode3/usr/bin/*4.0*; do sudo ln -s "$SRC_FILE" .; done
sudo ln -s /Xcode3/usr/bin/*4.0* .
@cullub Go to developer.apple.com, log in with your dev ID, choose "SDKs", scroll down and choose "Xcode", choose "Download", scroll down and choose "Additional Tools", search for "Xcode 3" in the search field.