Buyer release code

All,

Suppose I have 4 buyers in the purchase dept .

For these buyers there is only one purchasing group .

The role of buyers here is to create PO , that's all .

Less than 10,000 price Buyer shud release the PO and more than 10000 price Category buyer should release .

I will create 4 release codes for buyers .

Whether 1st buyer is raising PO , he only shud release it , the other 3 buyers shud not have access to release this .

How this can be possible .?

Please advise , this is urgent.If not clear I can again describe .

regards