Is Zakat on wealth obligatory for money invested in a real estate partnership that has been halted for four years due to unforeseen circumstances, especially given the resulting losses?

Zakat is obligatory on the money remaining from the project for all past years if it reaches the nisab. As for the money that an investor receives back from the sale of land, its zakat depends on their intention:

If the intention was to profit from the sale: The amount is treated as trade goods, and zakat is obligatory on it for each of the past four years, by appraising their share of the land and paying zakat if it reached the nisab. If the intention was for rent or residence: There is no zakat on the amount for those years; rather, a new hawl (lunar year) begins for it after the sale of the land.

Debts are deducted from the amount before zakat if it reached the nisab.