Old iphone4 as an ipod

I have an old iphone4 that hasn't been used in a while and it has been wiped. When I power it on it stops at the message about not having a sim card and won't go any further. I tried all the online tricks like hooking it to iTunes to get past it and that didn't work. The emergency call to bypass it actually calls emergency so that didn't work. I don' t have a SIM card, it is an AT&T locked iphone. So how can I just use it without the phone, just use wifi etc? I can't get past the no sim card screen.

Thanks... Gary