Apple Watch series 4 and IPhone 6 Plus

Good Morning,

I am looking to buy an Apple Watch series 4 for my partner as she has the iPhone 6 Plus, and to my understanding the Apple Watch series 5 wouldn’t work with her phone.

would the Apple Watch series 4 be compatible? Are there any functions that wouldn’t work?

I read that os 5 is compatible with the 6 plus but os6 isn’t.

please help before we shell out for the series 4.

many thanks in advance.