That has GOT to suck..... I cant imagine having to pay for internet and have a usage cap.
As for your question.... you're probably going to have to look into the Battlefield Section of the forum, or other Battlefield forums elsewhere.
Online games generally don't download/upload a lot of data. You should be safe on 4gb if you were ONLY doing online gaming.
I believe it's around 50mb per hour. There are 1024mb per 1gb so if you do the math...
If I did the math correctly, you should be allowed 81 hours per month more or less depending on the game you're playing.
lol at that sort of data rate, i'll run out of usage within a week
What ISP is only allowing 4GB per month? That's like 90 minutes (or less) of streaming Netflix in HD.
If it is a mobile hotspot, it generally won't work well (if at all) for multiplayer gaming. Most connected devices on a mobile hotspot will get a type 3 NAT, which won't allow player-to-player connections.