I see the picture on your last post and it's what I would call a bottom bracket. It's actually an assembly and they do fit where the cranks and pedals are situated as the crank arms and pedals are attached to them. There are two types of bearings for them, caged and press fit bearings. I don't know what type the RW4 has. Removing and replacing caged bearings is not an issue but removing and replacing press fit bearings is more difficult. I also don't know how one would find out what size of bearings one needs as generally the bottom brackets are purchased as a complete unit.
FWIW I just measured the frame for the bottom bracket on my RW4 and it's 183 mm. It was with a tape and not a calliper so naturally I don't know how close it is but it's probably accurate within a mm or so. The actual bottom bracket will be smaller as they have to fit inside. However the measurement of the bottom bracket frame is how one normally orders them.
I hope this helps and if I'm wrong about what you are asking, I apologize.