Sudden Behavior Change

I need some help. I have 4 cats, 2 male and 2 female. they're 3 and 4 years old, and I've had them all their lives. Two days ago, one of the males, Charlie (3) started hissing and growling at the other male, Dale (4). They've always gotten along in the past and I don't know where this is coming from. I have feliway plugged in, it doesn't seem to help. I don't have the option of putting Charlie in a seperate room and shutting the door. I live in the basement of my mother-n-laws house, and we keep the cats down here with us bc she has 4 dogs. Basically I have two rooms to live in, and there is no door to seperate the rooms. Does anyone have another suggestion that I can try? It's only been 2 days, but I'm already at my witts end. I'd appreciate any help that you can give

Sudden behavior changes often are symptoms of illness, possibly one of your boys is sick. Either the illness is causing behavioral changes or the scent of the ill is different and causing this response......I would watch for other changes and have a vet visit to rule out health issues as soon as possible.

I'm assuming they're both neutered.

So something has changed, you just have to figure out what. I would first think it's a health problem with one of them. They are indoor cats? If not, there could be a scent one picked up that is distressing to the other.

Another reason (less likely) could be another change with your or other people in the house and it's causing him distress. I hope you get it figured out.

It's possible one of them saw a strange cat outside and turned his aggression on your cats as they were 'available'. It usually takes a couple of days for that to pass, but not being able to separate them so they forget makes it a little harder, if that is what the problem is.

I'm sorry, I realize now I didn't specify that Charlie is sick. He does suffer from cronic throwing up, he has his whole life, due to complications when he was born. My main problem is that I have no way to seperate them when Charlie starts to get territorial. I'm looking to see if anyone has any suggestions on something i can do, or a product I can try, to lessen the behavior. After 3 years, I'm honestly suprised that it happened in the first place. I didn't think that I'd have a problem with the two boys being that Charlie's been with Dale since he was born. Also, Charlie has no problem with either of my girls, just Dale.