As for the differences between Gen 3 and Gen 4 pistols, as a generalization:
Gen 4 pistols have the different grip texture (known technically as “RTF3”), dual spring Recoil Spring Assembly unit (with frame and slide changes to accommodate), larger & reversible mag catch (not ambidextrous as some claim), and four back straps. Gen4 guns are built on the SF frame, resulting in a shorter trigger reach and shorter heel depth than the Gen 3 guns (about 2mm). The SF frames also use a smaller trigger housing mechanism. Gen 4 guns use a “dot” connector instead of the Gen 3 unmarked connector. Gen 4 guns have an updated trigger bar. All Gen 4 guns come with 3 magazines, whereas only the blue label Gen 3 guns came with 3 – white and red label come with 2.
Some Gen 3 models are available in a factory threaded barrel version (to support compensators and silencers). Some Gen 3 and 4 models are available in a factory ported version – although not all models are available in both generations. Some Gen 4 pistols are available in a MOS (“Modular Optic System”) version to support optics. Some Gen 4 pistols are available in a few more factory colors (including Gray, and BFG) than Gen 3 pistols. Both are available in black, FDE, and OD. These are all color molded frames with black slides (the only color Glock makes), trigger, mag catch, slide lock, and slide stop lever. Gen 4 includes color molded back straps. Some Gen 4 guns are available with front slide serrations and extended controls (mag catch, slide stop lever) as well as steel sights.
From a maintenance perspective, the recommended replacement threshold for the Gen 3 recoil spring assembly (RSA) is every 2500-3000 rounds. For the Gen 4, it jumps to every 5000 rounds.
Some will point to the fact that the Gen 3 has been around for years, and thus, is proven. Well, so has the Gen 4. Production on Gen 4 guns began in 2009. I think that’s sufficient time to be considered “proven”.
Some will say the “finish” is different. Glock made a change to the metal treatment process (which is applied to the bare metal, before the finish), starting in 2007 (and that transition was completed in 2010). That original process was known as “Tenifer”. The change to the newer Melonite process caused subsequent guns to have a finish with a slightly different appearance, but a higher HRc rating, which is good. It’s not about the generation of the gun that dictates the finish, but WHEN the gun was manufactured. A Gen 3 gun and a Gen 4 gun both manufactured on the same day will have the same finish.
To see more technical data for each specific model, including both current Gen 3 and Gen 4, see https://us.glock.com/products/all.