Summary of the situation : the reactor 1,2,3,4 have been struck by explosions, one pool with depleted fuel in it is on fire, radiation level are rising so much that safety of the personnel in the control room is seriously compromised and the cuve of the reactor N°2 is damaged. Source guardian.
Please do make use of better sources, and refrain from spreading misinformation.
There was no explosion at Unit 4. The fire in the spent fuel containment facility has been put out by 2am GMT.
Sources:
International Atomic Energy AgencyWorld Nuclear NewsUnits 1, 2 and 3 are at cold shutdowns, and being cooled of residual heat(
http://www.world-nuclear-news.org/RS_Cold_shutdowns_at_Fukushima_Daini_1403112.html). Unit 4 is in the process of being cold-shut. The pressure/temperature control system of Unit 2 apparently breached, releasing some radioactive gases, and increasing radiation to 8mSv/h for a short time, having decreased significantly by now. Main problem here is restarting the cooling process.
The most dangerous occurence so far seems to have been the fire, which released radioactive dust into the atmosphere, and at one time increasing the radiation dose in the vicinity of U4 to 400mSv/h. Keep in mind that it's not a sustained dosage, and simply the maximum recorded during the whole incident.
Most of the plume has been blown away towards the ocean.