i had upgraded and again downgraded to v3.
REVIEW:
*FF v4 Full screen apperence is Damm good
*Refresh button is beside of URL bar its bad idea to keep refresh button far away as IE has!
*For New Bie's its hard to find bookmark unless they know the short cut key as alt+b or they know to get it from settings sec
*All FF Addons are still are for v3 (this is the reason y i have downgraded)
Well, it's not launched officially as they're not forcing FF 3 users to update to FF 4. They're still testing everything. I'm gonna stick with FF 3.