Not too surprised, with how things are going in MoP I would of been very surprised if it took then more then a month or two to get 5.4 on the PTR after 5.3 was live.
Unless Blizzard makes a very sudden announcement, it'll be interesting to have the final chapter of an expansion all laid out before we know for sure what the next expansion will be. While I have a feeling a pre-BlizzCon announcement of Expansion 5 is within the realm of possibility due to the timing of everything (or at least more so then it has with previous expansions), we'll probably know a big announcement is coming at least a few weeks in advance.