4:1 is odds, so it's win 4 - loose 1, or 80% - 20%

So chance of loosing 2 in a row is 0.2 * 0.2 = 0.04

96% : 4%
divide both sides by 4

24:1

Winning 8 4:1s is 0.80^8 = 16%, or 1:5.25

If you want me to explain more I will, I don't want to aimlessly ramble.

Cliff notes: no there isn't an easy conversion for odds, you have to use percents.