Column A has a list of items, B has their cost. Column C states if the item
is to be purchased, and D states from which account.

So I need a formula that will add up the costs from column A only if that
item is to be purchased under a given account.

So I need Excel to add values from B4:B21 if C4:C21=Yes AND D4:D21="Petty
Cash" for each row.

It's frustrating, it feels like I'm on the brink of having it, but I'm just
not seeing something somewhere.