hub/ 4x4 question
 

Hey guys, I looked at a 94 Ford Explorer today, and I've got a question you guys might be able to answer. The centercap of the front left wheel, wasn't in all the way. So we pushed on it, it went in like its suppose to be but as soon as we took our fingers off it came back out. Could this be any sort of a problem/what could cause this?

Actually, I guess its the hub is loose? Does this mean it needs replaced or what? I'm clueless when it comes to 4x4s