Agression between cats - at my whits end.

Actually, I think you do need to start over.

Do you have two rooms where the two sets of cats can be separated and confined?
Do you have any vertical space for them? The more vertical space the better. Cat trees, window seats, a pyramid of plastic boxes with holes cut in them - shelving on the walls for them... (like this: http://www.thecatsite.com/forums/sho...d.php?t=201467)

Do you have Feliway? It is a synthetic hormone that mimics the "friendly" markers in cats' cheeks and helps reduce stress.

Right now, you ALL need a break.

Separating them and starting over is the best thing you can do in my opinion. Each set of cats will make the room they're in "their territory."

...and once you start over with the introductions, the more space you can have for cats to escape (like up - especially like the "wall of cats" ) so they can't be cornered will also help.

We live in a very small space with 7 cats (an RV). Apart from creating a ton of vertical space for them, we also did things like buy a lot of those concrete forming tubes - about 8" around - very thick - from construction stores (we got ours at Home Depot). We have them in hall-long lengths, so kitties have the option of passing each other and no one can block passage from one area to another.

I also think you may want to look into the use of Bach's Flower Essences for kitties: http://www.bachflower.com/Pets.htm

...but the basic idea of introduction is to help the cats' associate each other with "good" things. Rub the boys all over with several rags and rub the girls all over with several rags. Put boy-scented rags under the girls' food dishes. Put girl-scented rags under the boys' food dishes.

Play a lot with them - this also helps reduce stress. At the end of the play session, put treats down for them - for the girls on another boy scented rag, for the boys on another girl scented rag.

Also, cats learn best by association, not by correction. When you begin actually re-introducing them, I'd do it by bringing one boy into the girl's room for a supervised visit - then one of the girls into the boy's room. When it seems things have stabilized a bit, let one girl and one boy out into the rest of your home together for a day - put them in their own rooms for the night. Whenever either of the boys is near the cat they've been harrassing and they don't do anything aggressive, praise them to high heaven! Praise her to high heaven for not acting like a scaredy cat.

Make sure to rotate their toys a lot - and have new ones, their favorite kind, ready to give them during the supervised re-introductions.

Focus on the positives, do what you can to reinforce those with praise, start over, go slowly, and work on scent swapping to get the cats to re-aquaint them with each other but through positive experiences.

...and in the meantime, prepare your home for the four of them by creating absolutely as much vertical space as you can and whatever other measures you're willing to take to give them spaces and ways to get around so they can move around without being cornered.

...of course, the alternative is to just create separate rooms for the boys and girls to which they are confined at night.

Wow Laurie, what a bunch of GREAT advice.

When my boyfriend Dana moved in with Nova, we had problems where he would chase after Cleo II and she would scream and freak out too. Luckily it did get better over time, but when we adopted Azizi, Nova and Azizi became best buddies and nova taught Azizi to bug Cleo II as well. So every now and again, we'll hear Cleo II scream and know it's one of the 2 young boys driving her nuts, but luckily it isn't a common occurance and for the most part they live fairly peacefully. I hope everything works out!

Please do! Another thought.... litter boxes on each floor (once they're back out and about). Have some covered and some uncovered. Litterboxes can be a place where kitties feel unsafe. The rule of thumb for the number of litter boxes you should have is the number of kitties you have plus one - but I'd have two or three on the primary floors. When we lived in a house we had six cats and nine litter boxes.