Just for kicks, here's what the solution looks like in the Haskell programming language:

combos n ls = filter ((>= n) . length)
            . map concat
            . sequence
            . map (([]:) . map return)
            $ ls
[download]

(This is analogous to the Perl solution provided by hv earlier.)

Examples:

> combos 1 ["abc", "def"]
["d","e","f","a","ad","ae","af","b","bd","be","bf","c","cd","ce","cf"]
[download]

Here's the (elided) answer to your example question:

> combos 4 ["abc", "de", "fgh", "i", "jk", "l", "m"]
["ijlm","iklm","fjlm","fklm","film","fijm", ...
 "cehijlm","cehik","cehikm","cehikl","cehiklm"]
[download]

Cheers,
Tom