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Backstraps for those who have slightly smaller or larger hands. Dual spring. Enhanced mag release, lefty friendly. Different grip texture. Different slide finish. Very small differences.
Biggest, is if you want smaller or larger grip size than the gen 3. Otherwise, if gen 3 is cheaper get it. Gen 3 is just as good as the gen 4 for quality.
In most cases, the Gen4 comes with 3 mags vs 2 with the Gen3. That pretty much explain the cost difference, but the physical differences have been noted above.
Gen 3 usually had a better slide finish than Gen 4. Google Glock Frying Pan (Gen 3) finish vs Chalkboard (Gen 4) to see pics of the differences.
Also Gen 4 trigger bar has a bump on it that may contribute to a slightly heavier trigger pull but can be replaced with a Gen 3 trigger bar quite easliy.