Ring Fimal Circuits are basically two sets of conductors wired in parallel.
If you measure r1 + r2, you are in fact measuring two sets of parallel conductors, but in series.
If you had a Radial circuit 50m long and then decided to turn it into an RFC, all you would need to do is run another 50m cable from the Consumer Unit to the last socket.
So although you might use 100m end to end of cable to wire an RFC, the circuit is in fact only 50m long or only 1/2 as long.
Because an RFC is in fact two conductors in parallel, the resistance is halved another 1/2.
Multiply these two 1/2s together and you get 1/4.
Therefore when we measure r1 + r2 in an RFC, we divide by 4 to obtain R1 + R2.