CPU shouldn't be an issue, whenever I watch 4K videos on YT, my CPU usage is around 5%. I have i5-4430. Update your drivers, try FireFox beta - it has HTML5 support. Also check if you have the latest Flash player plugin.
EDIT : Yup - I just checked - 4K 60 FPS videos on YT, 5% CPU in Firefox Beta, 75-80% CPU in Chrome. Used latest FF Beta and Chrome. Interestingly enough - despite CPU load never reaching 100%, video was still choppy as hell. Seems like some bugged codec in Flash Player plugin for Chrome.