It only needs to be a small cast. In fact, this isn't a major skill in L4 - it's not like L5. The small hollow cast is part of the FHC major skill (rather than being one in it's own right) so even if completely omitted, I think it's .3. Really bent arms is also a deduction of course.
I stack panel mats under/slightly in front of the bar. Make two stacks, they should be able to go two at a time. Start with straight arm jump ups. It will take them a little while to figure how to do it with straight arms, but as long as you haven't set the bar too high (set it about chest height, don't want it too easy) they can do it. When they can do 10x with straight arms like that, then they do straight arm jump up, immediate FHC cast to return to the mat, jump up, FHC, cast back to mat. They do this 5x in a row. This also helps with learning to connect the FHC to the straight arm lever up action which will help with the kip FHC in L5. And it is good conditioning/endurance for them to do skills in a row because the L4 bar routine has pauses.