Using an inactive iPhone for iMessage?

I just recently got the iphone 5s, all set up with my number and everything. However, I was wondering If I could also activate iMessage on my old iphone 4 to text people using my number. when I activate iMessage on my iphone 4, it shows my number ticked and gray but then when I sign in, it says "Send & Recieve" and then no addresses. However at te top it still says Waiting for Activation but it's said that for a while. is there something I'm doing wrong or something I should do to get it working?