It appears the 4s is the only 4 on the list for iOS 8.
If I were to bet, there isn't enough free space to update. Here's an article about that.
" users are required to have nearly 6GB of free space on their devices in order to actually download and install the update. "
Looking around it appears you may be the second one to report this. I care, but I'm not there to see what's up.