Ok, 6S is cold on a heart lead (as is 6NT), and either will make on a black suit lead if the vulnerable 2H bidder happens to hold both the diamond ACE and hte heart king (a vulnerable stopper squeeze, run the black suit, if RHO comes to 1H, cash the heart ACE and Queen, if he keeps two hearts, exit the diamond). The question is how should this hand be bid.
Are you sure your partner takes 4NT as ace asking? If you bid an immediate 6♠ do you expect partner to bid 7♠ with the diamond ACE? Do you expect both 4NT and an immediate 6♠ to get you to grand when it is on (and do you want to be in grand if partner just has the diamond ACE given the bidding so far).

Help

Bidding..... to me in 4th seat...
P - P - 2♥ - ?