The plan covers both regardless of where you're located.
I believe thatPJNC284 is correct. At least my 4G plan is $30 -- no increase from the data plan with my old Blackberry (which wasn't anywhere near 4G!).
I also wanted to add that you might find it beneficial to turn 4G off when you know you won't be able to use it, like in your home town. This can make quite a difference in battery life in my experience. Apparently searching for 4G uses more power than having a solid 4G connection.