There are a lot of older threads with a ton of info (if you can search). There should also be Frequently Asked Questions (FAQs) threads that can walk you through almost everything you need to know on other sites. Here's a general overview:
-Operation: Stacked Tube Blowback (Spyder/Clones operate this way)
-Unique Features: Entirely enclosed operating system = no dirt/grime gets in!
-Barrel Threads: ACI F4/Griffin and Tippmann Prolite/Procarbine/A-5 (A-5's have a ridge so they should be the last resort)
-Powersource: Co2 or HPA
-Maintenance: Oil Regularly (Like Gold Cup Etc. that you would put on your timmy bolts). I liked to tear mine down after every day of play and hand spread oil accross all the o-rings. Then before every day of play I would place a few drops in the ASA when I aired it up.
-Upgrades: Are around, use a google search. Double triggers, Expansion chambers, "LP" chambers, and Venturi Bolts are available.
Here's a few more sources of information-
Reviews: http://www.thempn.com/reviews/f4/f4.html http://www.pbreview.com/products/reviews/1044/
And the best source of information: The Manual!
<- Apparently this site isn't welcome by PBN, just google search F4 illustrator