Gonna be my first 4g phone as well. I'm sure they'll throw in the SIM card for free, it's just $10, not much for them to lose.
The 4g lte sim card is part of the package. When you buy the phone they have to give you the sim card just like att/tmobile do. Even if you switch back to a 3g phone and then back to a 4g phone, they wont charge you to get a new sim card. So you'll be fine you wont have to ask them about it, that's the only way the phone will work properly.