My iPhone 4S 32 gig takes too long to charge HELP

My iphone 4s 32 gig is a great phone all around. It takes rediculously long to charge. I went from 19% to 40% in fours hours. I have never had this problem with any previous iphones. Is this normal? Or is this a hardware or software issue? Warranty issue?

I woulld appreciate any tips or advice on this problem.

Thank you