The 4 workers (2 each for each reactor, I assume) will measure radiation levels, temperatures and humidity, water level, and take pictures.
The torus (Suppression Chamber) of Reactor 2 is considered to have sustained a damage on March 15, though TEPCO says it was not an explosion. TEPCO and NISA both agree that Reactor 2 has released the most radioactive materials, some of which though may have come from the dry vent.
From TEPCO's handout for the press in English, 3/13/2012:
Field Survey on the semi-basement floor of the reactor building
of Unit 2 and 3 at Fukushima Daiichi Nuclear Power Station
In order to grasp the leakage route (damaged part) of the reactor cooling water in the torus room....