If you have `p` rows of 4 patches then you have `4cdot p` patches in total. Now if you add `y` patches to each row then you would `p cdot(4+y)=4p+py` patches in total.
It's similar as counting seates in a hall. If you have `r=10` rows and `s=16` seats in each row, then there are `10cdot16=160` seats and if you add 4 more seats to each row, you would have 20 seats in each row, meaning there would be `10cdot20=200` seats in total.
I hope this helpes.