As far as the differences between the two, I prefer the Gen 4 grip over the Gen 3 (i cant speak for the strap interchange but i just like how it gives a better grip in its material), but I currently own a G27 Gen 3 and simply put some Talon Grips on there and it has been great.
I love them both. Have not had an issue with any 3 or 4. Some do have minor issues with the Gen 4 but it is a very limited yet easily corrected prob. I can't honestly give a "con" on either based on my experience. Gen 4 you do get a 3rd magazine..... I shop for the best deal and do not care if its 3 or 4 Gen.