In almost 2 years of being a line holder, I have flown with the same captain for the whole trip (I bid 4 day trips), but have also had 5 different captains and three different FA's in one day. At our base (JFK) there are many captains on reserve, so it would end up being one captain for a leg or two, then he gets pulled off and you get a new one...
With regards to safety and familiarization between crew members, everyone is trained exactly the same, so everyone does things exactly the same (with regard to checklist, procedures, etc.), therefore switching of crew members does not have an effect on safety.
If you get a great CA or FO to fly with for 4 days, it's great! If you get a bad one, you may wind up having a really long 4 day.