Just got my Rezound two days ago and quickly discovered the same issue. 3G and WiFi fine - 4G a no go on Youtube app. Talked to a Verizon rep on the phone last night who had a model Rezound at his desk and poof, same on his. I think he was aware of the issue oddly enough through these very forums, and he pushed through something on it to make it even more aware on whatever end it needs to be known on.
Though it's not part of the topic, I'll also add that Skype video isn't working on my Rezound (properly), either. You have to hold the phone in landscape orientation in order for the receiver to see you "upright" and for you to see them correctly. When holding the phone in portrait orientation, the skype app does not auto-rotate, and I cannot find any settings to manually adjust it. Same issue with the Verizon tech's phone too.
So far those are the only two glitchy apps/services I'm noticing. Netflix worked fine, Paltalk, etc... so other than the phone being a HUGE BRICK with the extended battery in it, I'm decently happy with it so far.