I called IC about the choke size they'd recommend for shooting lead 4s (not Longbeards) out of an 870. They said .555 for any size lead. I expected them to recommend the .570 for 4s like most other choke makers. Obviously they know their chokes better than me, but does anyone have experience shooting lead through one? Goes against my understanding to constrict 4s that much.
Choke size isn't the problem here
Pellet size and Pattern density is.
Lead 4s are a poor choice in a 20 Gauge for turkeys. If you can get your hands on some 5,6,7, 7.5 shot you'll be much more effective with the 20.