Help! 2 of my 4 dogs seem to legitimately hate each other and their fights genuinely have me worried for one of their lives!

I have 4 small female dogs, 3 chihuahua/mixed and a small terrier mix. 99% of the time they get along perfectly and always play together. About 2 or 3 months ago the small terrier and the largest of my dogs, a long haired chihuahua mix which is at least twice the size of my terrier got into a nasty fight that left the terrier shaken up. We didn’t know the cause of the fight but it left me and my family feeling worried. Since then, they have had at least another 4 spontaneous fights. What we realized was that anytime a stranger comes to our house or is simply nearby and causes the 4 dogs to get excited and they start barking, that’s when for some reason these 2 get to fighting.

I took them out for a walk earlier today and it happened again but this was by far the worst one. As a strange man was crossing the street towards is, they got riled up and these 2 began fighting again. The big one bit my hand as I was breaking them up and she looked extremely menacing at the small one and even as I was carrying the terrier back home, she continued to jump up trying to bite the terrier that was in my arms. I will say that it does seem like the terrier instigates initially but just knowing her, I don’t think she’s ever trying to fight. The way she plays is generally but jumping on the others and she runs circles around them and barks but she’s just being playful (at least to me). I guess the big one doesn’t like that and she aggressively barks and bites her which ends up in a nasty fight.

I’m at a loss as what I can do to remedy this situation. Obviously I am keeping them completely separate from one another and I leave them in separate rooms when I go to work. Is there some training or steps I can take to make this living situation work? Giving away the big chihuahua has crossed my mind but I am very attached to all my dogs and can’t envision giving her away. However, if she does get ahold of the small one and no adult is around, I think she can genuinely leave her gravely wounded. I don’t know what to do.