Yes, I have no idea what kind of relationship they have, and I should have worded it differently. I'm thinking mostly of myself and how I feel. We all have that one issue that's very important, and you have to be on the same page with that one thing. Same thing with kids. If one partner wants kids and the other doesn't but compromises and has them - he / she is always going to be a little resentful of those kids. It's not like choosing a house or where you live. That's the only point I was trying to make. Like I said, I worded it badly, and apologize for that.
One of those "the grass is always greener". There may always be someone more compatible than your current partner, but if you're both invested in the relationship, you make it work even if you're not on the same page, paragraph, or line about certain topics. This may be the only point of contention (and it's in the distant future) whereas the rest of their relationship might be perfect and they might agree on and prioritize all of the same things otherwise. The OP has already said he is willing to compromise to keep the four current cats and accept them as part of her family, but if he has a different view of their future and how catly it ends up being, his partner should be able to listen openly about it and if needed, make compromises of her own.