Here's how to simulate the roll of a 20-sided die: enter
To simulate 4 such dice, enter the same formula in cells A11 (you can do this with copy-paste).
in cell E1. This is the sum of the four dice.
Maybe you can see from this how to get two more "dice" in cells F1 and G1, and their sum in H1.
I'll leave it to you to figure out how to use an "=if" to put the score of the game in cell I1.
You can then use copy/paste to generate 1000 such games in rows 1-1000 of your spreadsheet. I'll leave it to you to figure out how to get the average.
If you see a smiley above, it's because I entered a colon D and the system helpfully translated it to an icon I did not want. I don't know how to turn that off (he said, not smiling).