Firefox 4 has been released recently and it is not available via update manager in Ubuntu 10.04 Lucid Lynx and 10.10 Maverick Meerkat. The Ubuntu Mozilla Daily Build PPA provides it but it always the latest daily build and not the stable version.
For stable version, you can use Firefox Stable PPA. To install the PPA in Lucid or Maverick, just launch Synaptic (System>Administration>Synaptic Package Manager) and then go to Settings>Repositories>Other Software. Now click Add and paste in the following:
Close the Repositories window and then click reload. When it is done, search for firefox-4.0-gnome-support and mark it for installation. It should mark other required packages. Apply and wait for installation to finish.