According to the Bruce Beveridge blueprints (Hosted on this site), the forward staircase dome was oval, the aft was circular.
The dome may have been a feature added afterwards, but the staircase and funnels were always meant to be exactly where they are. This is integral to the ship's design all the way down to the keel and the boiler placement. Basically, the boilers determine the position of the funnels, and the staircase just slips in between funnel #1 and #2.
The forward grand staircase is just grander than the aft staircase. Among other things, it has a larger vestibule, and the dome covers part of the vestibule. If you think of each staircase as a stack of round balconies surrounding a courtyard, with a staircase attached. The forward grand staircase has a rectangular court (with an oval dome), and the aft has a square court (with a round dome). So it's just an aesthetic choice. It would be totally impossible to put a smokestack there.
The debatable 4th funnel would have been the last one, which is over turbine engine room. No smoke is generated there, so there's no need for a smoke stack.
Thank you, Iv been trying to convince people the forward staircase dome was oval. With the smokestack idea, I know it's impossible I was just playing with concepts. It would be interesting to see early concepts of the Olympic class ships, I imagine they'd be very different to how the final products looked.