When one of the four of you isn't there, it's tough for the remaining three.
If one of you is missing out of the four, it makes it harder for the other three.
この場合、次のような言い方ができます。
ーWhen one of the four of you isn't there, it's tough for the remaining three.
「4人のうち一人がいないと、残りの3人は大変だね」
one of the four of you で「あなたたち4人のうちの一人」
tough で「大変な」
ーIf one of you is missing out of the four, it makes it harder for the other three.
「4人のうち一人がいないと、他の3人にとってもっと大変になる」
out of the four で「4人のうち」
ご参考まで!