Well, I think the obvious answer to your dilemma here is two kittens
Sorry, I didn't mean to be flippant, and I know you have two already, but I wasnt being altogether frivolous. If you adopted two kittens at the same time, they could play with each other for company, and would be less likely to annoy the older cats. It can sound like utter madness, but it can also work really well, if your budget allowed (and I realise it's not something everyone can accommodate)
NWF cats are absolutely adorable, (we believe Moray's ancestry definitely contains NWF in the mix, as he's NWF in miniature with a personality to boot) and they have a well deserved reputation for being laid back and companionable cats, so any cats with that in the mix stand a good chance of integrating well with other cats, but that is of course a generalisation, and Bella may adapt just as well.
I think you have to listen to both heart and head here, and if you aren't in a position to take on two kittens, then you need to think very carefully about what you do next. It can be easy to rush in to acquiring another cat (I know - bin there, done that, bought the Gurlie T shirt
) but you need to be certain that if things don't go well initially, you can work at it to get it to near where you want or need it to be, otherwise you could be setting yourself, Bruno, Bebe and Barnaby Jones up for a very rough ride.
I could be very wrong, but I think you're nervous about your choice of Bella. How does she interact with her littermates? Is the lady you're getting her from someone you can talk to about your concerns, or is she just in it for the cash? (Not being judgmental here, just trying to establish how approachable she is if you were to voice your thoughts to her).
I dont know if it will help you or not, but I can share our experience when we got the Gurls. It was just after we'd lost one of our beloved duo, and my OH needed to have something to focus on. We went to our preferred shelter, looking for a male cat, but they had none. What they had were two absolutely traumatised 8 week old kittens who'd been dumped in a lady's garden in a cardboard box. They were absolutely terrified, and semi-feral. Hadnt been handled much, if at all, and they were wild. Their eyes were huge and hunted, and they hid all the time. Thankfully, acquiring them coincided with me having just come out of hospital and needing a couple of weeks to recover from an illness, which we then spent just quietly sitting in a small room with them, reading talking, and letting them get accustomed to us. It has taken 6 slow and quite painful months to get them to where we are now, which is that they are much more confident, and will come for some cuddle time, but very much on their own terms. If we need to pick them up to put them in a cat carrier to go for vaccinations for example, it's a full-on military campaign. They scratch, bite, hiss and spit. They go bananas. They will like as not never lose that instinctive fight or flight reaction now. Notwithstanding, I still wouldnt part with them. They're funny and engaging, and considering what they've been through, they're coming along just fine.
It doesnt seem to me that Bella is as bad as that, and I doubt you'd need to spend as much time socialising Bella, but you may have to spend some time with her on her own, if your other three aren't struck on the cut of her cloth. That's true of the NWF Mix kittens too though.
I dont know if that will help much, but I really feel for your indecision at the moment, and hope that you can think things through and come to a decision you're happy with.
