Player 3 should certainly be a part of your training plans. I'm not sure how much benefit you will get out of training IS since most of these players don't have strong enough ID to make them an inside option. Player 4 does have that base of inside skills, but his OD and passing are so low that you'll need to spend lots of time just to bring them up to decent levels.
If you train JR instead of IS, player 1 becomes a much more viable option. His potential is lower, but he's younger and will be worth more in merchandise revenue. Considering this, I see you getting the most value from training players 1 and 3 along with your main trainee.