Yeah, I never send an MMS unless I am on 4G. I never have any problems, using stock app. In all fairness though, I don't send a lot of them.
Many times, your phone will say it's on 4G when it's really not. To verify real 4G connection, I open my Speedtest.net app, and if it pings and finds closest server, and the button pops up "BEGIN TEST" then I know I am on 4G. It usually comes up just about right away if I am. If not, then I know that Metro is lying to me. lol
Sometimes then in that case, I have to turn off 4G using the AnyCut shortcut, just to get 1X. It's verry slow (esp. when used to 4G now), but at least it works for some things (browsing simple pages, email, maybe not a big MMS though).