Four hundred and thirty crew sounds like a lot of people, enough for four to hide among, yet wouldn't someone notice that there are four new faces (or at least two, since O'Brien and Bashir worked together, and Sisko and Dax were the other team), wandering around all of a sudden?
Didn't Lt. Watley, the one who flirted with Bashir in the turbolift, say she'd just recently transferred aboard? Maybe folks just assumed these others did too. Although there might've been some suspicion when they weren't seen again.
Anyway, my thinking is that the DS9 crew's interventions were minor enough not to make any substantial alteration in the timeline, so the variant branch they created would've re-merged with the original timeline after they left, and people's memories of the modified events would've faded or been blurred with their memories of the unaltered events. So time travelers who are only around for a brief period and don't have any substantial impact might tend to be forgotten.