WASM memories do have a hard 4GB limit, at least until the 64bit extension lands, but there's nothing technically stopping a tab from using more than that as a whole if the implementation chooses to allow it. I just tried creating a dozen TypedArrays of 1GB each and Chrome didn't panic, and the heap profiler shows ~12GB allocated as expected. Mobile browsers are much more strict about memory though so don't expect those to be so forgiving.