What size piping for a toilet stack?

MY old cast iron toilet stack needs replacing and the plumber is suggesting a 4 inch pipe but I think that the cast iron one was bigger than that.

Can anyone advise me as to what would be the best size to get? It will be servicing 2 toilets and I want to get most of the house sink drains to go into it as well.

All advice much appreciated

Your plumber is right, 4 inch.

It's the internal diameter you should be looking at; 4" is correct.

Thanks for the information .