4 = 2^2 therefore log2 0f 4 is 2

8 = 2^3 therefore log2 0f 8 is 3

16 = 2^4 therefore log2 of 16 is 4

32 = 2^5 therefore log2 of 32 is 5

etc.

So,

if you are dividing by 4 then you would put 2 (which is log2 0f 4) in the input register R13

if you are dividing by 8 then you would put 3  (which is log2 0f 8) in the input register R13

if you are dividing by 16 then you would put 4  (which is log2 0f 16) in the input register R13

if you are dividing by 32 then you would put 5 (which is log2 0f 32)  in the input register R13

etc.