What's the usual protocol for taking him to crowded places, can you follow that?
Or do you just avoid crowds with him altogether so he won't be overwhelmed?

I seriously hope its not the latter because being overwhelmed is part of life and we can't be sheltered from life...

Why not have a designated quiet place for him and a trusted adult? A place with little to no traffic?
Or why not have him leave directly after the viewing? He doesn't need to sit through the service.

Better yet. Ask him what he wants, he's 4 he speaks. If he wants to attend, let him, if he's ready to leave, take him elsewhere. Simple.

I really think your fear of doing it wrong is making you overthink this ALOT and you seem to be going round in circles. I say let the boy decide and just be there in whatever capacity he may need you.