Usually playing 1-2 months / year over 4G.
Game + mumble is in my experience roughly 100 MB / hour. I used to get by on a 10GB / month datacap but then I was using it super conservatively (and also overcharged it a couple of times, you should make sure that you get a service that just reduces your speed instead of one that charges you a shitton for any MB over the datacap.)
Depending on where you live 4G isn't that bad, I had ~20 more ping and some minor packet loss at times. At least on the swedish net 4G can get shafted during peak hours (around 7-9 ) when everyone else gets home and starts netflixing though.
If you can then find someone that already got a 4G-router and borrow it for a couple of days to see if it's acceptable.