Indoor cats vs Outdoor cat

I think you might have a little cat fight while you're not home lol

A stray will require socialization, and most new cats will require a proper introduction as cats are territorial and its natural for the current cats to try and defend their turf from an invader. Young cats may not have developed those behaviors yet (usually kicks in around 2 years old IIRC).

Most keep them separated by a door and first start with scent swapping using towels and then use a divider and play/feed both, and take things slowly.

Four cats is plenty IMO though, heh.

I would be vet checking the stray before allowing free roam of the house with your cats, you'll also need to do a slow intro until they are comfortable enough to be left alone.

(and four cats isn't plenty! )

You have three female cats? Have fun! IMO girl cats can be real pain in the butts when accepting new cats. But unless you have a couple of clashing dominate cats, they may be able to learn to ignore each other. My older female cat does not like the other cats in the home... though strangely she'll sleep near them and if they don't look at her will groom some of them. It's a tentative acceptance that relies largely on them ignoring other to keep peace - and for the most part they've settled in after a while. They'll still fuss from time to time but the worst result is that someone will get a scratched nose - usually one of the male cats that decided to bug my old girl. (never the younger kittens, they know better)

Do slow proper introductions and hope that over time everyone settles into their routines.

Do make sure outside kitty gets tested for FeLv and FIV before coming in. FeLv can be a problem with the other cats in the house hold, though FIV can be managed.

Looking at your cat pages... you had one outside called Oreo that you lost? If I read that correctly, I'm sorry. It's definitely best to have the outside kitty in, for their safety and just because it hurts so much to lose them.

Stitches has the sweetest face, btw!

Before you bring that cat in, make sure you go to the vet, just to make sure the cat doesn't have anything that can be passed on to your cats.

Females tend to take longer to accept a new cat/kitten. First thing I would do is isolate this cat, have him vet checked, shots, worming FELV tested and neutered.

After he is cleared, then you would introduce him very slowly to your other cats. I'd pick out the most friendly of your cats to start. The one may never accept him inside the house - and if it causes problems with use of the litter pan, spraying, etc., then you will have to rehome him.

And he may not like being a totally inside cat either. There is a lot to take into consideration and hope things work out.

Our females are hissy about new intros too. Spooky takes 6 months - 1 year to get over it. She's just hissy about it though. She'll bop "newbie" on the head if they bug her.

All of this advice is great. ALL of our cats are indoor-only, rescued as ferals from outside. We don't have room to separate, so going to the vet first for us is an absolute must. And as one of our kitties is immuno-compromised, if any rescues are sick, they have to stay at the vet in boarding until they're better. And we found that this helps IMMEASURABLY to making the kitty coming inside be HAPPY about being inside. They're so relieved to be out of the vet that being inside a home is like no big deal or something. If you can afford boarding for a few days or a week, it's something to consider, if you're worried about kitty wanting back outside.

But to summarize:

1) Get kitty in a crate and go directly to vet. (Talk to vet and see if you can do a flexible appointment). If not neutered, have neutered before bringing him home if he's healthy enough. Don't even bother with a fecal (unless there's reason to suspect giardia) - assume he's got worms. We have the vet treat them with Revolution. (Treats round worm, fleas, ticks, flea eggs).

2) Get a room ready for kitty. Release kitty into room.

Do as much as possible for him on a schedule - food, change water, clean litter - and then otherwise just have family members spend as much time in there as possible. If he's happy, play with him. If he's scared, don't try to interact, just read out loud, fold laundry, work on a laptop, do whatever you can, and just let him get used to all the new scents, sounds, &etc. Give him as much time as he needs to settle in. Cats are all about territory, and intros will go much more smoothly if he's comfortable in his space.

As to introductions...

We always start with scent swapping. We rub the new cat all over with a bunch of clean rags and then do stuff like put those under the food dishes of our kitties, put treats for our kitties down on them after a good play session... and vice versa. Get them associating each other's scent with "good" things.

Do the same for him - get him associating the scent of your cats with "good" things.

Then start by opening the door a crack (with you there). Do the kitties hiss at each other? Is he hiding? Is he curious? Trust your instincts. Start slow - don't overwhelm him. Bring in just one cat at a time, for maybe 15 minutes. Have a wand toy or treats ready to go, and don't focus on them "meeting" each other so much as doing something with them together. Or letting him watch you interact with your kitty.

There are no hard and fast rules... I'm sure it'll work out just fine.

This is a really wonderful thing you're doing!

Yes make sure you have the outdoor cat tested and vet checked before they are allowed to interact with your other cats.

It's funny that people said females take longer accepting a new cat, because one of my females LOVES newcomers, she will cuddle and groom them and etc... She also loves dogs, when my brother-in-law brings his dog over she will lay right next to him.

There definitely are the exceptions! I've read about other "newbie" friendly females on here - but I think they're outnumbered.