iPhone 4 can't FaceTime with iPhone 6

Hey guys,

So my friend with an iPhone 4, for some reason, can't connect to FaceTime with me. I have an iPhone 6. I also tried to FaceTime them using a 6 plus and that doesn't do anything. It will say connecting, and that's as far is it goes. I told them to go on wifi, which they did, and that also didn't help.

Anything would be useful.

Thanks,

Jon