I have both, in 9mm.
I like both & could quite easily live with either as a carry pistol.
The 3.6 is simply "more" of what I want, without getting into too much "less" in capability.
When I bought the 4, it was with the intention of moving "down" to it from the full-sized 2.0 9mm I've been carrying.
When the 3.6 was announced, it was simply a better fit, for me, going forward.
My thinking was, as I get older, I prefer a lighter gun, but won't lose too much by going to something like a single-stack .380. Not yet, anyway.
The 3.6 is shorter than the 4 in both length & height while still carrying 16 rounds.
That little bit of length difference between the two leaves the 4 in the vault now.
The Shield EZ .380 sits as an eventual carry option when I lose more upper body strength, but I'm not ready for that yet.
And then I got the 2.0 .45 Compact in for testing.
Same size as the full-sized 2.0 9mm.
Same size, as it turns out, as my beloved Colt Commander & carries 11 .45 ACP rounds, while staying about five ounces lighter than the Commander with its 9 rounds.
Hmmmm…..

Denis