DS had a bad cold now almost completely lost his voice.

[JonesTheSteam]

He is only 4.

He's been off nursery (attached to local primary) since Monday. Much better in himself today though. He still has a slight cough. He sounds more ill than he really is when he tries to speak (IYSWIM!?)

Is there really any point sending him in tomorrow if he can barely speak?

No, not if he can't speak. He probably needs to rest and school will wear him out.

If he is still like it on Friday I would think about the doctors as there might be a problem with his throat.

DS1 had something similar before Christmas although his throat got better after the cold came out properly and his nose started to run. Sounds like yours is the other way round, but DS had it for 2 days before getting better.