If I were going for 120 bucks I would used Leaved. My reasoning?
Leaf=Noun (singular)
Leaves=Noun (plural)
Leafed=Verb, but sometimes used as Adjective and when used as Adj, the definition is: 'having leaves; leaved.
Since leaved would imply plural as in the noun Leaves and we are talking of plural leaves on your shamrock (four), I'd bet on Four-Leaved.
I know the dictionaries give both leafed and leaved, so as sarumite says you are not going to get an answer with any degree of certainty.
But that's the way I'd reason it and bet accordingly.
Good Luck.