Gen4 has replaceable backstrap inserts for the frame (large & medium), or the frame can be used without an insert for the smallest possible grip circumference. Gen4 has a nested dual-spring recoil spring assembly, similar to what was created for and used on the Gen3 subcompact models (G26, G27), and internal mods to frame, barrel, and slide to accept this larger-diameter spring assembly. Magazine button is reversible for left-handed operation on Gen4s. Exterior gripping surface of frame is covered with a pattern of little pyramid bumps to assist in gripping the weapon; Gen3s had roughened sides of the grip-frame and deep checkering front and back (sometimes called grenade checkering), except for RTF2 models (the last major tweak to the Gen3 lineup), which were similar to the new Gen4 frame surface (but the Gen3 RTF2 frames had finer/smaller bumps than the Gen4s). Gen4s run $50-$100 more than Gen3s in the same caliber/frame/slide configuration (retail price, based on my local observations).
"Placement is power" -- seen in an article by Stephen A. Camp
(RIP, Mr. Camp; you will be remembered, and missed)