Common prefix of two strings, upto first four characters

commonPrefix.hs

-- Common prefix of two strings; up to 4 characters
commonPrefix :: String -> String -> String
commonPrefix "" "" = ""
commonPrefix "" _ = ""
commonPrefix _ "" = ""
commonPrefix s1 s2
	| s1 == s2 = take 4 s1
	| otherwise = cp s1 s2 []
	where
		cp s1 s2 acc
			| length acc == 4 = acc
			| head s1 == head s2 = head s1 : cp (tail s1) (tail s2) acc
			| otherwise = acc