What is the ruling on Zakat for money saved for the purpose of marriage or entering into a project, then used to purchase land in installments for building a property, over the past four years?

Zakat is obligatory on wealth if it reaches the نصاب (minimum threshold) and a Ḥawl (lunar year) passes over it. Zakat for past years is calculated by estimating the amount saved each year. If you are unable to determine the exact amount, then disburse what you believe most likely clears your liability. As for land purchased without the intention of trade, there is no zakat due on it.