Ok i found an Old 9.6 V nimh Rc battery that still works. Could it damage my M4 if I plug it with a mini battery adaptor just to see if my 8.4 V battery was the problem?
Edit: ahhhh just saw that my adaptator was a Mini to Large adaptator...

Too bad But The 8.4 V battery didn't work with the rc car so I'm pretty sure the battery is dead.