well im 3g is jailbroken...basically just want my contacts..is it the same way? how it the jailbreak affect it?All you have to do is backup your existing 3G and then when you plug in your iP4 then just sync from backup. You will find this also restores everything you had on your 3G. So messages, settings and app information.
i thought if you use itunes it syncs from the mac to the iphone. can you go from iphone to mac? then back to mac to iphone?If you use something like Exchange, Google or MobileMe then it will automatically sync contacts. If they are only on your phone then if you can get iTunes to Sync Address Book to contacts.