Same problem for me. Captain 1. I had 3/4 and couldn't find the last guy. I just dropped the mission and moved onto something else. These bugs are really irritating, especially when they NEVER get fixed.
I just found some old bug reports I sent out last year. The bugs are still in the game and happening on a regular basis. Sloppy.
I can't believe this hasn't been fixed yet... I tried diplomacy and failed (only a lvl 6 LC), ran past the guy and was attacked by him. Killed him and killed apparently three other hostile romulans but still missing the fourth. Wasn't sure what to do because the map shows a bunch of hostiles behind the wall that should be moved/blown up/something. Oddly enough, my Boff I beamed down with me somehow managed to jump through the wall. I was hoping I could but no luck with that... I guess I'll restart the mission or something then.
I just had the same problem, myself. I was supposed to take down 4 groups, but there were only 2. I also logged out and waited a while. When I logged back in and continued the mission, all was well. Took out 2, and then could continue on.
Zombies are one of my least favorite species of the Galaxy. I must destroy this one and remind you Humans to open a new thread if you need to post on a topic that has not seen the light of day in over 30 days.