How do I convert my iphone4 into an ipod?

If you have been using it as a phone and have cancelled service or transferred service to a new phone, then you don't need to do anything. Without active phone service it is effectively an iPod Touch at that point. To get the best battery life, turn on airplane mode and then re-activate juat wifi so the cellular radio is powered off.

IF you leave the SIM in it, it can also still make 911 calls in the USA (FCC requires that any device, with or without a service contract of any kind must be able to dial 911 if it is within range of a compatible network).

Set it up through iTunes. I presume it has already been activated, so set it up as a new device, delete any extra Apps other than the stock Apps so you have plenty of space for music, select playlists and sync. I have been using my old 4S as an iPod for a couple of years with no issues.

Cheers

Pete

If it is disabled you will have to restore it in iTunes to use it for anything.

Forgot passcode for your iPhone, iPad, or iPod touch, or your device is disabled - Apple Support

It would have been helpful if you had of mentioned that first up. Do you not have a passcode?

Pete