NFHS question

[oppool]

This question came up on a recent discussion and I was not clear on what the correct answer should be.

B4 is due up to bat but S1 who has not been in the game bats Unreported. S1 takes 4 balls and goes to 1st base. The defensive Coach now comes out and want a BOO. You inform the Coach all you have is a Unreported Sub which is nothing but a warning at this point. The question is who is on 1st? Does S1 have to remain in the game and now becomes a reported Sub for B4 or can B4 now get on 1st base and S1 is still not considered being in the game. Does the coach have an option?

Thanks

Don