Hello
This is the correct system behavior for planning strategy 40.
The PIR quantity is used to plan your material in advance, that means, to plan when there is no sales order. When a sales order is created, it consumes the PIR, that means, it replaces the PIR.
MRP will generate a planned order of 400 because this is the sales order quantity minus the stock.
Best regards
Caetano Almeida