The easiest way to do this on a very large scale is to just have ONE and ONLY one formula that applies to all cells, so you are copying it all the way down the sheet one time.
I'd do this by looking at ROW() to get the row number, and more specifically, using MOD(ROW(),5) to get a row index that goes 0-4, with 0 being the blank row.
This formula will look back up to previous rows so you will have to "prime the pump" for the first 3 rows manually:
- Cell C1 will be
=A1+B1 - Cell C2 will be
=A2+B2 - Cell C3 will be
=A3+B3 - Cell C4 will be
=SWITCH(MOD(ROW(),5),1,A4+B4,2,A4+B4,3,A4+B4,4,C3/C1,0,"")
Then copy this cell, C4, down the entire length of the sheet.
My specific formula assumes there's no headers and the values start in row 1.If not, you just have to move the formulas to the right SWITCH indexes 0-4.