A four-way riser or a saddle:
If you cannot get the fall required go outside and take a look at the gully dish where the shower waste is discharging at the moment but is just too high. If it is clay you will have to dig it up and pull out the up stand out of its P-trap. Having done this you will be able to fit clay to 4-inch (100 mm) PVC adaptor and rising up out of this place a four-way riser after drilling out the entry point you wish for the shower waste. Then continue on upwards to the new PVC gully dish to take the drain to its completion point and transfer any remaining other waste pipes into it.