I live in China and am in the market for a new phone. My HTC Desire HD is getting rather cranky in its old age.
Investigating 4G here, I've found that the mainland uses its own band, which no other country uses. Everything I've read about it says that even if you buy a phone in Hong Kong, its 4G won't work here. Now, I have seen some phones advertised on HK websites that say they can use both HK and mainland 4G, but I haven't been able to confirm that on any manufacturer sites. Not to mention, they were substantially more expensive.
Luckily there are some Chinese brands that are really improving their wares. The Huawei Ascend Mate 7 is one, and the XiaoMi Mi4 is another. I'm still wary of buying one though, mostly because, while it will have English, everything on it will be set up for Chinese consumers.
I'll keep looking into it because ideally, I'd really like to have a Nexus 6, or Droid Turbo with 4G that works here.