Those errors usually mean websockets are not proxied correctly, make sure your nginx setup is correct. If you are using cloudflare you need to disable acceleration. They don't support websockets on smaller plans.
No promises on 0.4x, but I would say under two weeks - if you need email functionality etc the most stable version of NodeBB currently is 0.3.2 so you'd have to drop down to that. Either that or get up to date and live on the Edge 🙂 (Hopefully not a production forum!)