YOU are on lead against the bidding shown. What would you lead, and why? YOU may know the rules about leading the top of a sequence, or leading partner’s bid suit, in this case diamonds. But stop to think.
You need four tricks if you are to defeat 4 ♠ , and there is not the slightest chance, on your diamond holding, that your partner will be able to win more than one trick in the suit.
On the other hand, you’ve got the trump ace so are bound to win a trick when declarer tackles drawing them.
Lead the ♥ 8. It may not win the trick, but when you win with the ♠ A you can lead a diamond in the hope that east will have just one winner in the suit, and be able to give you a heart ruff before it is too late. This would give you three tricks anyway, but another may develop from somewhere.
To lead a diamond initially would merely give away a defensive ‘tempo’.