Applying a formula to a large specific amount of cells in a column

I feel like this is a noob question but I can't figure it out and I was wondering if you guys can help me out.

So I am basically doing a sum function for a row that has four columns. (a + b + c + d = sum in e)
I know you can drag the highlighted square to go down and also you can us CTRL + SHIFT + END KEY (or DOWN ARROW) but the thing is the file is way too large for excel to handle. I found out I have a 32 bit excel program which limits the computing power of the program (company comp) which can't handle 750,000 + rows all at once and will crash or not be able to save. So the only thing I am trying to do is highlight every 20,000 rows and then adding the formula. Is there any quicker way of doing this? Such as I paste the formula where I can specifically say cells A1-A20,000 apply then after it computes do the next batch or so.

Thanks!