Introducing pairs of cats

The person I'm dating has two cats, both female, and I have two male cats. Because she lives in a very small space, her two cats have been staying with me. They are not fond of my boys, though, and I'm looking for more ideas on convincing them to get along. The girls are kept separate from the boys. I have a two bedroom apartment, so the boys have the smaller, extra bedroom, and the girls share the main bedroom with me. Each pair alternately gets to roam in the living room as well. The boys get more living room time since they have the smaller bedroom and since they were here first. The girls are sisters and are three years old. One of my boys is 3 years and the other is about 15 years.

I have a Feliway diffuser (though it's almost empty now) in the main bedroom. They have had a few face-to-face encounters, but all of those have resulted in one or both girls growling, and a few have ended with my younger boy attacking one of the girls.

What are some additional strategies I can try to get the cats to live more harmoniously? The girls have lived with other cats in the past. Both of my boys came from a shelter with free roam rooms, so they have had experiences living with others. The girls are the ones hissing and growling, but my younger one is the one most likely to smack or pounce on someone. My older one is pretty oblivious and tends to just want in the room to eat the girls' food. I'd be grateful for any tips on integrating the two pairs.

How long have they been living with you? I just added Lily (1.5 years old) about 6months ago for my older cat Callie to hang around with (17 years old).

When I first got her, Callie was hissing and growling. Lily would antagonize A LOT. This lasted 4months and they gradually are getting along.

Any cat in a new situation, (of course exceptions happen) are going to be scared, and looking to see where they fit in. There is going to be hissing/growling for a while.

If you are able, i'd say maybe more time with all 4 of them together a day, with your supervision. Less supervision if they are declawed. I can't say i've seen any cats actually bite deep wounds on another cat, but of course it can happen.

The ones with claws have the advantage here. So just do your best to watch how they act and get as much time together as possible. Time is what comes into play here.

They've been with me for about a month and a half...maybe two months. In one of their earlier face-to-face meetings, my young boy scratched one of the girls just above the eye, so I'm a bit cautious about giving them too much face time. I think the girls could adapt well to my older one, since he doesn't seem to antagonize them, but my younger boy seems a bit aggressive at trying to get close to them. He wasn't initially--we were impressed because he was actually quite calm. But since the scratching incident, we've been watching them very carefully and limiting their interaction. Do you think it would be better to bring the boys into the girls' room, or to just open the doors and let them all roam? Also, is it better to try to acclimate them all together, or just bring in one new cat at a time?

ETA: All four have their claws, which I just trimmed again yesterday.