Fourth Bridge 2, I would assume 1 is similar, has the engine in a compartment alongside the galley, in the centre of the boat and under the outside helm area. There is a couple of doors in the galley that allow access to the engine for reed filter checks etc. From memory is was very quiet from the outside apart from the occasional splash and burble from the exhaust which was along the port side just below the outside helm position.
Cannot really remember what the noise was like inside as we always cruised it from outside even in a slight downpour. We just put rain coats on and got on with it. One downside of the centre cockpit sliding roof boats is that you have to put the roof up to stop furnishings from getting wet and then it can feel warm and claustrophobic.
I assume Fourth Bridge 1 and 2 are similar although Fourth Bridge 2 joined Bridgecraft later as it came from the neighbouring yard Anchor craft when they retired from hiring boats.