To double then bid a new suit must show a strong hand. Also the bidding up to this point is giving you several clues. Dealer is doing nothing but passing and the other opponent prempted to 3♦s. So should have between 6-11 points at best. Your 10+preemptor's 6-11 if that many is only 16-21 points. With 19-24 points out there who has them more importantly would dealer pass if he has an opening or close to an opening hand. Now we are ready to consider partner's hand. To double then bid a new suit at the 4 level shows an opening+ hand and long ♠s. With that in mind I would now look at 5♦s as an exclusion keycard blackwood bid and that would enable us to place the contract in the correct spot either 5, 6, or 7, ♠s depending on the response. :)