I am looking at an m4a3 for my first ar and I saw that cdnn investments had the bushmaster patrolmans carbine for $799 which seems like a great deal but i was wondering what the difference is between a regular m4a3 and the patrolmans carbine? Also, who has dealt with cdnn investments and are they still running that deal? Thanks a lot

Bushmaster's Patrolman's carbine has a 16" barrel, and a removable A2 flash suppressor.  The M4A3 that I think you are talking about uses a permanently attached "izzy" flash suppressor.  The barrel length being the difference.  You cannot mount a normal bayonet on the patrolman's carbine, but it is more mod friendly because of the removable flash suppressor.  As far as I know, Bushmaster doesn't use true M4 handguards.  
I have the Patrolman's carbine, it was my first AR.  I have enjoyed it a lot, and found it very adaptable to changes.