There have been some pretty blazing GR 150 4-man runs recently, with the US and EU hovering around the 4 and a half minute mark, but they still haven't been able to beat the sub-4 minute best over on the Chinese servers. The video below is actually of an earlier attempt, which seems to have been removed from the Chinese leaderboards, as they finished the rift on 3 minutes and 52 seconds a few weeks ago, while the current Rank 1 on the official Chinese D3 site is sitting at 3 Min 57 sec, with different players, and is more recent. So, we're not quite sure what happened there, but we can still get an idea of the speed involved here:

Diablo III D3 by Starym