You can qualify your columns inside the panelGrid using columnClasses. The following code sets different width and stick the cells content aligned to the top-side.
<h:panelGrid columns="2" style="width: 100%" columnClasses="forty-percent top-alignment, sixty-percent top-alignment">
<p:panelGrid columns="4" columnClasses="ui-grid-col-2, ui-grid-col-4, ui-grid-col-2, ui-grid-col-4" layout="grid">... </p:panelGrid>
I like more the answer from @NwawelAIroume as it sticks with the CSS framework and doesn't add any extra CSS (which maybe one day break responsiveness).