The total value of the four payments in the first year is R100 so X=R100 and X/4=R25. The total value of the four payments in the second year is R200 so X=R200 and X/4=R50 etc. The memo has used the annuity formula X[((1+i)^1-1)/i^(4)] where i is the effective interest rate but you would be able to use the other method using X/P and i^(4)/4 too.
The reason that the memo is accumulating each four payment period for a year is to create a perpetuity in arrears of 4 year payments. Each year is considered as one payment and then each four year period is being considered as a "group" of payments that keeps repeating.
Please let me know if you require any further explanation.