A couple of reasons. A year or so ago I decided that I wanted to pick up 4-string again, after a decade + of nothing but 5s. Then at the beginning of this year I decided that I was going to completely retool my playing from the ground up, studying with Ant (and trying to get rid of 40+ years of bad habits). I had discovered that I could cover my gigs on a 4, and doing the ground-up was going to be a bit simpler on a 4. Since I use the RA to cover a lot of my jazz gigs, and most of those tunes were done on an upright, having a 4-string RA helps me think more along upright-style lines. In this case, less is more.
The other issue, though less of a big deal, is that my 5-string is 35" scale and I've gone to 34" for everything (except for a 33" scale E-C 5'er that will show up this summer).
My Deep 4 is damn near perfect. It is just a joy to play. My 5 is awesome, but a 5, with wider neck and another string, is more to handle. Certainly not a deal breaker and I have and will happily gig a 5-string. But for now I'm all about really getting a 4-string wired inside and out.