Chances are, if the construction is somewhat modern, that the main sewer line is 4". Many toilet lines are 4". Tubs are smaller, like 1-1/2" or 2". To switch places, you will need to replace the fittings on the main line. The main line could be iron. Some older construction could have the p-trap under the tub setting in the dirt and assembled with some sweated lead. I've seen the tub p-trap in dirt in houses built between the 30's and early 50's. If the main is plastic it will be fairly easy. If its iron, you'll have to rent or buy a tool to break the main line. Since the pipe pieces fit inside one another, and it was assembled from the bottom up, when you cut it, you won't be able to re-assemble it without a rubber splice. This is a tubular piece of rubber with a hose clamp on each end. You can find pretty good directions for this sort of work in a basic plumbing booklet.