I wouldn't recommend locking yourself to an older version of UE4, because of the many fixed Epic releases on a regular schedule. Moreover, our integration releases are in sync with UE's released. This means you will not have the new features that were introduced in The Wwise UE4.9 integration (like Linux and Mac) in the 4.7 branch.
So to answer your question, you will need to use the 4.7 Wwise branch, but if you do, you will not get any updates to the integration.
Note that we are planning on being a real UE plugin (no engine code patches), so you should be able to recompile the integration yourself with a different version of the engine. This should be available soon.