I’m not 100% positive on this, but I believe Sprint said that WiMax can handle simultaneous voice and data. I think I do remember though that this was still being “tested” so it might not be available yet.
It does stay on with 4g, I was using easy tether and received a call and my service did stay on my puter. Also works on the phone when I'm on 4g I can make and receive calls while browsing.