why is there a link to upgrade to firefox 4 even after upgrading to 4
Firefox may not have updated you to the new Firefox Start Page.
If you check the address bar, does it have an address similar to this?
That's the Firefox 3 Start Page. The new Start Page is:
Give that a try (type or paste that to your address bar and press Enter) and if you like it, you can make it your home page. See How to set the home page | How to | Firefox Help.
about:home -- not impressive
It's useful if you don't have anything better...
If you prefer the full Google experience, you can make http://www.google.com/ your home page without the Firefox 3 "skin" and not be bothered by that message any more.
yep, playing with that now, lots of gadgets...