Yes, this is somewhat old data. But I haven’t found one source that has everything one should know about the two generations of Glocks on the market. So I’m making one. We’re going to break down each difference here, and then talk recommendations at the end.

Grip Configurability
The Gen4 Glocks come with four backstraps: medium, large, medium with beavertail, and large with beavertail. They can also be used with no backstrap. The Gen3 comes in just one size. It’s very roughly the same size as the Gen4 with medium backstrap. So it will fit most, but you might get a better fit on the Gen4.

Grip Texture
The Gen4 Glocks added a reasonably aggressive grip texture. Gen3s are smooth sided, except for the Gen3 RTF2, which also comes with some good texture (but is harder to find). If you’re looking for stippling work, the Gen3s have a bit more material to work with in their grips that you can shape and play with.

Grip Accessories
The Gen3 doesn’t have possible backstraps to make things difficult. So there are more options for magwells for competitors, especially if you’re looking to tune the weight of your pistol to get the perfect balance between heavy to absorb recoil and light for transitions. Some magwells for the Gen4 can be had made for a backstrap. Otherwise, you’ll be modifying them to fit. It’s not very hard, just something to note.

Mag Release
The Gen4 has a bigger mag release than the Gen3, and it’s reversible for lefties. It’s easier for the small-handed to push. Mods abound for both platforms.

Trigger
The Gen4 has a somewhat different internal path for the trigger bar to take. Stock Gen4s (that aren’t 34/35s) come with a different connector than the Gen3s to compensate. If the same components are put in an otherwise similar Gen3 and Gen4, the Gen4 will have a trigger pull about half a pound heavier. As always, components are easy to mix and match to get the pull you want. Trigger components, including trigger bars, from a Gen3 will work in a Gen4 without difficulty. If you stick with the Gen4 trigger bar, remember to apply lubrication to the bump on the tab that engages the drop safety plunger. It will improve the feel of the trigger. Always lubricate between the trigger bar and trigger connector.

Barrel Fit
From Gen3 to Gen4, Glock changed something about the barrel fit. As a result, while drop-in match barrels will generally improve the accuracy of a Gen3 Glock, they will provide at best no measurable improvement in a Gen4 Glock. The OEM barrel actually does better than some aftermarket drop-ins for some loads. So don’t waste your money simply for accuracy if you have a Gen4. Note that both generations of Glocks will see improvement from a professionally-fitted barrel.¹

Recoil Spring Assembly
Glock changed from a single-spring recoil spring assembly in the Gen3 to a dual-spring one in the Gen4. Not much difference to feel in 9 mm, but it improves hotter calibers like .40 S&W. Note that competitors interested in an aftermarket guide rod and spring kit will either need an adapter or a guide rod designed specifically for the Gen4. This is a part that does not interchange.

There you have it. Overall, I think the Gen4 is a little better in 9 mm for most people. Some custom builds might be easier to do on a Gen3. It’s not a huge difference though, so don’t sweat it.