Looking at lofts of many iron sets, I notice that there is usually a 4* gap from PW-5, but a 3* gap from 5-2. Anyone know why there isn't a 4* all the way through?

I had my irons adjusted starting with a 49* PW and 4* increments all the way through and am liking this gap better in the long irons.

Loft gaps can vary as it depends somw what with the design of the irons. Cavity back irons will launch the ball higher than blades, due to the CG being lower in the cavity back design. The IMPORTANT thing is to have EQUAL yardage gaps between your irons, NOT equal loft gaps. I hope this helps to answer your question.
And Welcome to the site, it's good to have you on board.