I sent Cait to Abernathy farm as I couldn't send her to Sanctuary... Now I can't find her anywhere... In Workshop mode it says there are four people but only the Abernathy's are there... I tried looking back in the Combat Zone and reloading but no luck...

Try waiting (or sleeping) a day or two then look again in Abernathy, the Combat Zone and where ever your were when you sent her. Baring that, if you're on a PC you can type this in the console:

prid 79249

moveto player

Not been to Abernathy Farm, but is there any sections of freeway nearby?

I only ask, because when I managed to lose Nick Valentine I did eventually find him. I'd sent him to graygarden, but the game had spawned him on top of the broken freeway section which passed over the edge of the Settlement Boundary. There's no way to access that through walking alone. Had to build a multistorey staircase to get up there and move him.

No it's just an open farm area with one building and a electric pylon...

Another option is to build a town bell at Abernathy, ring it, and wait awhile. If she is in fact at Abernathy, she should show up.

*Edit* Also there is a big flat area under the pylon on the roof and I have heard stories of people spawning on roofs. You might want to check that.

I may build the bell...

Did you check that little trailer northwest of the farm? She might have shacked up out there with a raider since you have been ignoring her.

No she not in the shack... I built the bell... it says there are six people at the farm and only five showed up... I've waited a game week to see if she'll come back... I've been checking the Combat Zone but still no luck...