You can get cheats for gta4 by typing in a certain number into your phone for example the number for weapons is:486-555-0150
yes, gta4.net has all the cheats, not many cheats are in the game though.
Certain Cheats disable certain achevments.
no aeroplane cheats :'/
yes you can but sadly with cheats
no, you can't
look on gta4.net
you have to dial phnoe #s
type the cheat on your phone in the game