[Shugart23]

Bidding has con 1s-2d by partner-4h - all pass. You have K J 10 4 of diamonds and are going to lead one of them. Which one do you lead?

[P_Marlowe]

Assuming I dont have anything else worth while, the king, in an attempt to keep the lead,
and hopefully after seeing dummy, I may have chance to switch.

[DavidKok]

It depends on your agreements. We would lead the jack from this holding. The most important part is to not mislead partner, and to telegraph our holding.

With a suit so weak and apparently not that many values on the auction I expect partner to have 6 diamonds more likely than not, so we're only getting one trick in the suit (if that). Our carding choice doesn't matter too much since the ace will win the trick - the main question is whether we might wish to stay on lead with the king. For me the possibility of partner misunderstanding and getting a wrong count of the hand is more important, and I will make our systemic lead.
Keep in mind that this auction calls for an active lead, and we are not going to promote any tricks in diamonds. Strongly consider picking a different suit.

[apollo1201]

Without the rest of the hand it is hard to tell but probably K followed by a likely C switch through dummy and up to partners values, before déclarer discards losers on dummy’s S.

Having 6 HCP max in D, partner rates to have some other honors.

Of course, the sight of dummy might suggest sth else…if I am still on lead!