Biting 10 times in 4 weeks is extreme - I don't blame the school for threatening to exclude. I bet every parent in the class is aware and terrified it might happen to their precious 4 year old. I would be.
When DD was about 2, she had a biter in her nursery. She came home with bite marks on her skin where I could see the indent of every tooth of the biter - top and bottom jaws. It happened 3 times. Still furious about it. Still want to kick him to fuck. Nursery refused to give me the name of the child. DD told me it. AND when he did it, they gave him special 1:1 time, which (to me at the time - I think differently now - rewarded him for this behaviour), but they took him away from the group and gave him special attention.
FF 10 years, and DD starts secondary school. There's a very, very autistic kid in her class. She doesn't know or remember, but it's biter boy.
And for me, everything drops into place.
Not saying your GS is autistic, but (as others have said) maybe he's frustrated about communication.
Don't know about where you live, but round here, you can't get a diagnosis (unless it is very, very extreme) before the child is around 7 (because our CAMHS says that the developing brain changes so much and so quickly).
So you are stuck with containing until you can get a referral for a diagnosis. If it's that.
I get this isn't helpful, and I wish you good luck. there are support groups out there for parents who have children with SEN. Google. Ask your local council (the one with responsibility for education) and they should be able to signpost you. These are groups which exist to support parents and carers.
Good luck!
But, also, don't go into school all defensive ("he's a lovely boy" etc). They'll know and see his good sides. Go in offering to work with them. Because then they are less likely to exclude and more likely to support you with the next steps.