Multiplying by 1/r gives the PV now assuming that the perpetuity starts in 1 years time.
Here the perpetuity starts in 4 years time, which is 3 years later. So multiplying by 1/r gives a value 3 years later from now i.e. in 3 years time. So we need to discount for 3 years to get back to a PV ‘now’.