"Running a 4" drain" is laying a 110mm (4") drain from a suitable point on the existing sewer, under or around the building, to the location of the proposed new WC.
I really cannot believe the architect has not investigated the existing drains properly before doing the drawings, first rule is to know what's there before starting! As others have said, if any part of the proposed build goes over an existing shared drain, the job suddenly gets very complicated and expensive!
Ian has offered a possible solution, you need to ascertain depth, size and direction of flow in the drains before doing anything else, and if the existing chamber is not suitable to connect into, allow for adding another chamber where the new drain connects into the existing. This may sound like a lot of aggravation, but compared with a Macerator, it is still the preferred option for many on this forum.