Use old iPhone 4 as MP3 player only

Hi

heres a question for you guys.

i have just upgraded to a new iPhone from an Iphone4, I would like to use the old iPhone 4 as an MP3 player but I need a sim to activate, is this true.

If i use my current sim to active the old iphone4, can i then remove it and use it in my new phone?

Thanks for looking and any advice you can give

Eric