The Gen 4 is the same width as the Gen 3's as far as I can tell. However with the back straps on the Gen 4, if you use no strap (which is what they call small) you will have a smaller grip than a Gen 3. But that is front to back smaller.
I have shot and carried Glocks a lot, and of the 4 I have been through my current Gen 4 is my favorite. I don't see much point between Gen 4 and Gen 5 unless you hate the finger grooves, but at this point I would pay a little extra for a 4 over a 3.
09 Lincoln MKZ AWD 91k - M1 AFE 0w30
12 KIA Sedona 86k - Edge 0w40
09 Ford Focus 129k - GTX UltraClean 5w30
00 Chevy Lumina 195k - Supertech FS HM 5w30
04 Jeep Grand Cherokee gone but not forgotten...