Pre-ordered it? I've already played it (PS3, in the UK)
The hopefully non spoilery verdict thus far:
I like it as a game. The plot's continuation after the end of AC III is interesting, didn't expect that one bit. The most disconcerting thing is how you're dumped into the plot without any guidance of what you're able to do - instead of all the previous games giving you a run through of abilities and progressing through them, you're just dropped into the storyline fully skilled as a pirate.
Mechanically, combat seems to be much like AC III's, this may be good or bad depending on your perspective, lock picking appears to have been removed as a mechanic (which is awesome, I hated that about III), but the gameplay and feel harks back more to the likes of AC II, you don't have the map gradually filling in as you explore (you have to scale the viewpoints) and we're back to running around on rooftops, something more absent from III, I found. If you didn't like the ship to ship stuff in III, you may want to skip IV somewhat...
IV is certainly very pretty, and much more appealing to look at than 3 was, because you don't spend a long time looking at snow so much.
If you never played the series before, IV is not where I'd suggest you start. Storywise it's an ideal jumping on point but the mechanics and the expectation that you already know all the controls etc. make that very difficult. If you've played III and enjoyed III, IV is dialing it up to 11. Highly recommended stuff there.
Ah, I wasn't sure what platform you were hoping for it on, but yeah, it's already out for some platforms in some places.
As far as the whole 'treating players like adults', that could easily go both ways. On the one hand, being dumped into the storyline without any guidance is great and immersive - if you're familiar with all the controls and expectations, and with the 'fourth' game in the series, it could be argued that you should be.
On the other hand, the way the story has flowed through the previous games, you were never at a good point story-wise to jump into it unless you played from the first game, or at least had an idea of what went on in the first game onwards. But story-wise this is a great jumping in point.
My take on it is that if you're familiar with the series already, being treated like an adult and not lead by the nose for a bit is fantastic. I was more taken aback by the fact that we've had 5 games where you were lead around by the nose and suddenly we're not any more. I didn't expect it, and that was the problem. After having gotten around that idea, it works so much better because you don't have to learn how to be awesome, you're already fully tooled up on awesomeness.
(I will admit, there is some direction about ship combat the first time you do it but only because it's not entirely identical to III's ship combat, though anyone who prevailed at III's should be able to pick it up anyway)
But if you're not, you might find it a real headache to pick it up because of the otherwise high learning curve.