useless sabotage is right you cannot move purchases between Apple IDs.
What you could do is to authorise your laptop with your old Apple ID
Store > Authorise this Computer..
Then transfer all the old purchases from your old computer to this one so they are all in the same itunes.
Connect up your iPhone 4 to the new machine. Yes it will ask you to erase. Go ahead and erase then as all your music is on the iTunes for this machine you can resync it but still use your new Apple ID for future purchases.