Here are some things that I've learned with tunnel time. Tunnel is not required to do AFF, but it can be very helpful. 4 minutes is not a lot in the tunnel.. some people get a stable position right away, and some take 10 to 20 minutes of flight or more.
The key is to relax and have fun. Listen to what the instructors tell you. The more you are worried or tense and not having fun the less you will learn!
As for learning previous to going into the tunnel, when I want to learn something new I go to the IBA youtube channel and watch the videos (and maybe practice a move out of the tunnel) but then go in and listen to what the instructor is teaching me.
http://www.youtube.com/playlist?list=PLFA9176BAACA9B59F
Make sure you don't watch too many videos. Maybe watch up to video 3.
Anyways, have fun and let us know how it goes! As with anything in skydiving the more time the better...