Less parts are compatible between Gens 4 and 5 than you would think. Barrels of Gen 4 and 5 G19s are compatible. Also mags are compatible as you would expect. I believe recoil springs are compatible in the 19s. Sights also are compatible. And grip back straps I believe are compatible. I do not plan to swap any parts between generations, I will be getting Gen 5 replacement parts for use with my Gen 5s.

 

Magazines, the Magazine release, Grip plugs, and sights are the ones that I know for sure are compatible between the two gens.

 

Plus mag catch spring, channel liner, spring cups.

 

There are things you can do like put a Gen5 trigger bar and mechanism in a Gen4 frame. You have to use the Gen5 slide backplate with whatever slide you use. It will drop the striker and reset etc. Is it safe I don’t know? The key to compatibility seems to be the different slide backplate.

 

Here's a good reference-
http://looserounds.com/2017/09/04/gen5-glock-stripped-internals/

 

The Gen4 and Gen5 Locking Block is the same for any particular model.

The Firing Pin Spring is the same.

The Connector is the same.

The RSA for a Gen5 G26 is the same RSA for a Gen4 G26.

The Gen5 Trigger with Trigger Bar plus Trigger Mechanism Housing should not be in service with a Gen4 Slide with its Firing Pin Safety. The FP Safety rides up on a polymer ramp and rail on the right side of the Gen5 TMH as the slide retracts for any reason. The Gen5 FP Safety is properly ramped to ride the right side of the Gen5 TMH. The Gen4 FP Safety is just a circular plunger that is not designed to properly lift and ride the right side of the Gen5 TMH.

Approximately 70 percent of Gen4 parts are NOT compatible with equivalent Gen5 parts.

 

Hickok45 does an extensive discussion of this in his video on the Gen5 19.