There is a vapor source that appears to be coming from unit 4, as viewed from TEPCO cam 1. You can see examples of heavy emissions from this area Dec 22, 2014 here:
The atmospheric emissions from unit 4 have been oscillating, some days more, some days less. I don't see a definite pattern, but what I do see is that even during lower-emission days there are constant emissions from units 3 and 4.
What I am focusing on today is the constant trace of emissions from unit 4, which purportedly had all the fuel taken out of its spent fuel pool (not sure of status of equipment/machinery pool). Why does unit 4 continue to have significant emissions if the fuel is gone?
TEPCO cam 1 is positioned so that you only see a slice of unit 4 but there have been red/purple emissions and steam from that area constantly for the last few weeks (if not longer):
Let us look at this screenshot from cam 1 taken today. You can see some light emissions from unit 2 and from unit 4. Only a slice of unit 4 is visible but that slice is the source of the red/purple colors and emission halo, as illustrated in the second cropped screenshot (you can see the 2nd link above for another example):