If creatures an opponent controls are dealt lethal damage at the same time that Phyrexian Obliterator is dealt damage, those creatures will be destroyed before that player chooses permanents to sacrifice
You'll still be subjected to a coin-flip on every Stitch you cast or copy, but you're bound to win a few flips if you just cast the card enough times
Well, thats the short of it
However, you can consider playing fewer than four copies of a card if: It gets worse in multiples (such as legendary creatures) It's only useful against certain matchups It's a win condition, and your control deck will have time to dig through the deck and find it You're stuck choosing between two cards which have equally strong, similar, but slightly different effects (you could play two of each) It's a slightly worse version of a card you have four copies of already, and you're including it because you want even more of that effect
All I had was basic land, mountains, 20 of them