(Yes, I made the screenshot deliberately a bit too wide to have the crawler transporter in the image.)
Only 3 mobile launch platforms were made, and with only two launch pads, more than 3 don't seem necessary. So why the effort of building 4 bays - maybe next to each other to avoid the loop-around? Was the program budget already cut after the commissioning of the VAB? Was there an optimistic future use envisioned where they'd be launching Saturn-V rockets every other week?
Question: what was the design decision driver to make the VAB with 4 bays?