After your partner bids notrump, a bid of 4NT is a quantitative raise, asking your partner to bid 6NT with extra values. Therefore, it is impossible to use Blackwood in such a situation. The solution to this problem is the Gerber convention.
         The convention is this: after a 1NT or 2NT bid by your partner, a jump to 4 is Gerber, asking your partner for aces:

иии NT — 4 4:     0 or 4 aces.
  4: 1 ace.
  4: 2 aces.
  4NT:     3 aces.


         When is 4 Gerber?      A bid of 4 is Gerber when it is a jump-bid immediately after your partner bid notrump. In particular, 4 is not Gerber if your partner has just bid a suit, and it is not Gerber after 3NT.
         Note, however, that a jump to 4 is Gerber even if you or your partner has already bid clubs:

         EXAMPLE:     4 would be Gerber after 1 — 2 — 2 — 2NT.


         Asking for Kings.      After you partner reveals his number of aces, you may bid 5 to ask for kings:

иии — 2NT — 4 — 4 — 5 5:     0 or 4 kings.
  5: 1 king.
  5: 2 kings.
  5NT:     3 kings.