Is there a word for 'four times as much', analogous to once, twice, and thrice?
Not according to the Oxford dictionaries:
No, there isn't.
protected by RegDwigнt♦ Jul 12 '11 at 22:54
Thank you for your interest in this question.
Because it has attracted low-quality answers, posting an answer now requires 10 reputation on this site.
Would you like to answer one of these unanswered questions instead?