I'm still using MX 4.4.5.1000 on win 8.1 and I've updated my UA as follows:
Mozilla/5.0 (Windows NT 6.3; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Maxthon/4.4.5.1000 Chrome/42.0.2311.135 Safari/537.36
I don't know whether it is right or maybe there's something wrong/less, but so far it works fine to open YouTube, Gmail, and playing some flash games on Facebook.
PS: I don't see any UA update for MX 4.4.5.1000 even in old forum (There are only for MX 4.4.4.3000 and MX 4.4.5.1200 beta)