A Glock 21 works fine, but I don't have personal experience with converting a gen 4 model. Possibly there is a recoil rod/spring assembly that is already made that would work. If not, have you looked into the small bushing made by Glock Store that will fit into the recoil rod hole in the front of the slide? If it would work, this would allow you to use a non captured assembly from Wolff.
Any gen 4 glock can use a gen 3 recoil spring assembly with the addition of a little bushing at the front of the slide. That is very common and done by a lot of people.
It works for both captured and non-captured gen 3 assemlies, even the stock gen 3 spring (but why bother?).
