i'm not sure i understand, but i'll assume you mean that instead of buying 4 at once, you could buy 2 at a time? i think it might depend on where you're buying/looking. on a site such as expedia or travelocity, you dont' know how many seats are actually available, so buying at two different times might mean that you don't get to sit altogether, or even get the same price. we had this situation last time where we had a total of 7 people, and expedia only lets you purchase 6 at a time. so my friends purchased 6, and i was online too, went immediately to buy 1 for myself, but the price had already changed.
things might be different if you are on an airline's website, but i don't think you'll encounter savings by splitting your purchase - you might just increase the anxiety if you end up on different flights.