I can get five cards to cover every letter but 'v': Market Square, Bazaar, Jack of All Trades, Young Witch, Explorer. So you could insert your favorite village as a sixth card to get an equally good list. I believe a five card list is not possible:
'z' has to be either Bazaar or Plaza.
'x' has to be Expand, Explorer, or Taxman.
'q' has to be Masquerade, Quarry, Squire, or Market Square
'j' has to be Junk Dealer, Jester, Jack of All Trades, or Journeyman (interestingly they all start with J).
None of those cards have any of the other three letters in them (i.e., none have z and x, or z and q, etc.), so we need one from each list, and none of them have 'g', 'h', 'v', or 'w' either. So the fifth card must have all four of these letters (in addition to whatever else is not covered by the particular choice of four cards). A quick search through all the cards reveals no card has all four of these letters, so at least six are required to cover every letter of the alphabet.