> with 4 bytes added for itself

Why one would want to do that? Only reason I can think of is, so that values < 4 can be reserved for future users, like new versions of client-server that agree on 64 bit payload if the first 4 bytes are zero or alike. But there are better ways.

I'm guessing so it's never zero and can't be confused with something else, like a bunch of NULs. There are a few messages that's don't have a payload, e.g. ParseComplete.