The last person to leave Boiler Room 4 was probably trimmer George Cavell, who testified at the British inquiry. He described working to put out the fires in Boiler Room 4 (which was his usual post). Water gradually entered the room from below until it got to about a foot in depth, at which point Cavell and the others working there left. Upon reaching Scotland Road, Cavell, for some reason, thought everything was all right (in the words of the examiner) and went back down into the boiler room briefly. He did not reach the bottom because of the water, and he did not see any other crewmen there. He left and went to the boat deck. The scene he describes, with two boats left on the starboard side, places his arrival on the boat deck at something like 1:35 to 1:40 AM.
There are only a very few people who survived to describe conditions below decks as late as 1:30 AM. That is almost fifty minutes before Titanic was completely submerged, and it is even a long way off from 2 AM. But by 1:40 there were only four wooden lifeboats left, plus the collapsibles, and as you know all the boats were gone at 2 AM and the collapsibles were the only hope of survival.
As for the 'cargo shaft', there were two cargo hatches plus a coal hatch in the forward part of the ship. These wouldn't have been a 'shaft' like an elevator shaft. They're actually just a series of holes in the deck, with a raised border around the opening (a coaming). At the top deck there was a hatch cover, a heavy one, and I've just learned that there is evidence from the wreck that it blew off when the bow hit the seabed.
It is not really Boiler Room 4 (BR 4) but Boiler Room 5, the one forward of BR 4, that represents the change from relatively slow to rapid flooding. Taking a step back from that, BR 6 represents the death-blow; flooding the three holds forward of the boiler rooms plus BR 6 places the ship's waterline above the tops of the watertight compartments. Once BR 5 started filling with water the ship was already doomed, and with BR 5 full the bow would be well underwater.
Try this - float an ice cube tray in a sink and gradually fill it with water from one end. You'll see how it's 'waterline' gradually lowers until, at some point, it starts to sink of its own accord. Of interest is that it doesn't matter how fast you fill the 'watertight compartments', but how much water is in the tray. The amount of water in a ship, not the amount of damage, determines how fast it sinks. All this is theoretical though, and it assumes there is no further damage to the ship...
Now, as for the lurch, which is attested to by many survivors and happened around 2:10 -- that was most likely the ship beginning to break up. The first funnel collapse occurred shortly after that. The break was further aft than BR 4 (more like BR 1). After that, survivors describe more signs of the ship breaking up. The accounts vary widely -- but remember that the scene was in almost total darkness and that anyone still alive outside of a lifeboat was in a struggle for their life that they would almost certainly lose.
Well -- that's rather long, yes, though I think I can say I did give you one clear answer -- Cavell, if his testimony is accurate, was not only the last person to leave BR 4, but one of the last survivors to have been below decks.