I have both. My P5-3 is noticeably faster outside (transient yaw wind is a major thing). I successfully converted my P4 to track. My naked track P4 (sans bottle) with the same wheels and tires (trispoke and disc, record clinchers) matches the the P5-3 outside speed, only inside.

Is the P4 UCI compliant?

Is the P5?

Both are yes, position and equipment. The bottle and brake "stiffener" are off the P4 and it has the UCI compliant seatpost, although I believe the rules now make the original seatpost UCI compliant as well. The P5-3 was always compliant as is.