It's only really a problem if you use the IDs in the same system. It's highly unlikely that you'd link 4B schemas into a single binary. And anyway, if you do have a conflict, you'll get a linker error.

Cap'n Proto type IDs are not really intended to be used in any sort of global database where you look up types by ID. Luckily no one really wants to do that anyway. In practice you always have a more restricted set of schemas you're interested in for your particular project.

(Plus if you actually created a global database, then you'd find out if there were any collisions...)