What is the best way to make the purchase accurate, knowing that you have to manage 04 months lead time to receive the goods?

There are many variables to consider when managing04 months of replenishment lead time such as lot sizing, Customer service level, manufacturing lead time, inventory in the total supply chain, production schedules and strategies, economic order quantity, back logs, rough cut capacity planning, transportation planning, fill rate and many other variables from inventory and production planning perspectives. 

The best strategy is to (i)comprehend distribution resource planning (DRP) with master resource planning (MRP) (ii) join master resource planning to master production schedules (MPS) (iii) have Master production schedules connected with rough cut capacity planning (RCCP).