Is zakat due on the down payment of an apartment, or on its total price, or is it not due until after possession, when purchasing an apartment for real estate investment and receiving it after four years?

The amount you paid in advance for the apartment is not subject to zakat, as it has transferred from your ownership to the seller's. As for the apartment itself, if it is for investment purposes through rental, zakat is not obligatory on the asset itself, but rather on its income if it reaches the nisab and a full year has passed over it. If it is intended for trade, then its value must be assessed at the end of the year when you pay zakat on its price.