Are you planning to keep the Kindle or are you returning it?
ETA: Okay, then the next thing you need to determine is which is more important to you, the mobi files you already have or the ePub files you already have plus ePubs you'll acquire in the future. Amazon owns mobi so Sony, Nook and Kobo, in particular, aren't ever going to support that file format. Sony, Nook, Kobo and other ePub readers do however support borrowed library e-books. If you can borrow the titles you already have in mobi, that might be a deciding factor. If not, the only device you'll find that meets all your other criteria isn't going to be an e-ink reader at all. You're then talking about a mini-tablet with an LCD display. If audio isn't important to you, you don't need 4GB. 4GB less the amount used for firmware, equals about 3,500 e-books. I've been buying e-books for over five years in both ePub and Kindle format and my total of the two is about half that. If I ever needed that capacity, I could delete something to make room for something else or, in the case of my Sony Reader, I could supplement that with an SD card and a memory stick pro duo.
Honestly, I don't know a whole lot about Cybook or BeBook because they weren't available here when I bought my first Sony Reader in 2006. I fell in love with it then and I've had other Sony models since with never the least little hardware problem. I currently have PRS-350 and PRS-950 and I gave my dad a 350 in October. I've never had a problem with any of them and really, Sony is the quality, trendsetting hardware manufacturer in the bunch. Nook, who even knows who makes that? It's got last year's e-ink screen so it's overpriced anyway. Kobo is pretty low on features though I use their store for most of my e-book purchases because they've got more coupons than any other.
And now, Kindle. I have Kindle 3 only because it was a gift and I should have returned it while I had the chance. I'm on replacement number three and it's got a hardware problem right out of the box too. It's incredibly chintzy and, in spite of the new Pearl e-ink display, the keyboard and cumbersome menu tree are just a pain in the butt. The only thing Kindle has going for it is text-to-speech but that's usually not enabled by the publishers anyway and I prefer to read, not listen.
I would recommend that you check out the Sonys on display at Best Buy or a SonyStyle Store and have a close look at the features and the intuitiveness of the user interface, which wouldn't be possible were it not for that amazing new touch screen technology. I don't know about the return policy at SonyStyle, but it's 30 days at Best Buy. If you decide to buy one, get it at Best Buy. It may go on sale there again soon. Keep an eye on the prices at at Borders and B&H Photo online and Best Buy will price match them. In spite of whatever mobi content you have, I seriously can't imagine that you'd want to return the Sony once you took it home. Just make sure you've registered Adobe Digital Editions and the rest is super easy.
ETA: One thing I forgot to add; you can read your existing mobi content on your computer with Kindle for PC/Mac or on a mobile app such as iPod Touch. But because ePub is the more universal standard, I wouldn't buy any more mobi formatted content and I can't see that what you already have would justify a specific reader just for that.