"Don't do that" would be the first response.
Unless you are absolutely 100% certain that every single one of the recipients has a fast broadband connection and you are dead sure that none of them is currently on the road and, say, reading email via an international circuit switched mobile phone call.
4MB for a party invitation is ludicrous.
Send a couple of lines of text, one of which is a link, and put the pretty bit on a web server.