I am having a similar issue - the rate dropped dramatically (by a factor of 5, looking at my bandwidth readings, not actual tweet counts) on 15 Jan 2014.
This is using the Sample Stream.
I am using Twitter4j v3.0.5.
No code or config changes were made over this period at my end.
Perhaps compression has been enabled on the stream, which wasn’t previously enabled…? (Given that I have been looking at bandwidth not tweet counts…).
UPDATE: the overall number of tweets received per day is approximately the same as before the change, so I can only conclude that compression has suddenly started working (which is nice)!