Let’s have a plumbers perspective. A four inch sewer pipe will not be completely full with water when discharging. It may only have an inch if that, flowing. It would be no problem for a rat to walk up a sewer especially when it is graded at 1/4” per foot drop per most plumbing codes. Easy to walk up. The only obstacle would be the trap in the toilet which are usually not glazed so the surface would be good for a rats grip.