Since firefox 5 has been released firefox 4 seems to have been removed from the mozilla repositories (i.e. force version cannot be used).
closed as too localized by Jorge Castro, Marco Ceppi♦ Jan 3 '12 at 0:01
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In essence, you can't. There aren't that many major differences in the versions, and infact, if you use Ubuntu 10.04 and up, you will be upgraded to a newer version of Firefox automatically anyway.