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:
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: