Modding my speakers power question

I got an old dock for mp3 players and its powered by 4 AA batteries. Its got the 4 batteries laid side by side can i splice a usb chord to power this and how would I do this. could I just hook up the positive and negative to one battery part? I dont think this would work because the other 3 wouldn't be hooking up the positive and negative. Copper wire connecting the grounds and hooking up positive and negative to one?

Thank you in advance.