how can I unlock an iphone 4 without sim card?

Not sure what you mean by "unlock"? If you just want to use it as an iPod, and it was already activated in iTunes, then you need no SIM at all. Just enable airplane mode, then re-anable just wifi (turning off the cellular radio that way will make the battery last a little bit longer), and use it as you wish.

If you meant carrier unlock it, that is not necessary to use it as an iPod Touch.

If you've erased the iPhone 4 and need to re-activate it in iTunes, that requires a SIM be in it - it can be any SIM (as long as it is the right size) and only needs to be present for the minute or two it will take to activate it in iTunes.

You will have to get hold of a suitably sized SIM from somewhere, just for a couple of minutes. Activation does nothing to the SIM itself, so perhaps you could borrow someone's just long enough to activate your iPhone 4, then give it right back to them?

If the iPhone shows as needing activation, then you need a valid SIM, though you do not need to have a contract with the cell carrier.

http://support.apple.com/kb/ht3406

Regards.