It CAN be done. It takes time.

Dogs will make friends in a matter of moments; butt sniff and they are best buds.

Cats take thins slowly. You start by bringing him in and keeping him segregated. Spare bedroom works well. He has his on food and water bowl, litter box of course, a few toys to keep him busy, and a bed and blankie.

After a day or so, you put the 3 cats segregated, and let him have the roam of the house (use the bathroom to help make the swap.) This allows him time to get used to the smells, find his way around, without any harassment from the others. You may only have him our for a few hours, and then put him in the bathroom, release the trio and put him back in the spare room.

Have blankets for the trio. Swap the blankets so now they are all getting used to the scent of the other cat(s).

Do this for a while till he seems comfortable in the house. Oh and clip everyone's nails. Then while you are home and available to observe, let him out. Have a water squirt bottle ready, and feel free to break up any fights. Hissing, back arching is to be expected, I don't usually squirt or that, just observe, maybe talk them through it.

It may take 3 to 5 weeks, but that will seem like nothing once you've all been together for years and years!