As you’ve probably already heard or read, there will still be some Firefox 4 betas before the final version 4.0 is released. Mozilla foundation has moved the roadmap so the final version is planned no sooner than early 2011.
Currently, the latest version of beta Firefox 4.0 is 4.0b7 or a nightly version 4.0b8pre.
I think the main reason for shifting the release is that the code is not complete yet even if it has some new features. The second main reason is that there will be not enough compatible add-ons (remember, FF4 will use a completely new logic for add-ons which will make the old ones unusable), which might push even loyal Firefox users like me toward Chrome.