Delegation in PO release strategy where the buyer can select the approver

Hello All,

The business requirement is as below:

Say user S1 is the approver for release group R1 and release code A1. As per standard workflow the PO flows to the work space of S1 when the respective release strategy gets triggered. Now a standard instruction is given by user S1 that in his absence user S4 can release his POs (the instruction is out of SAP). While creating the PO the buyer should have an option to select S4 in place of S1 so that the PO goes to S4's work space and S4 approves the same. When S4 approves the PO S1 should get an email alert that S4 has approved the PO on his behalf along with the PO no.

Is this achievable and how? Your suggestions please.Thanks.

Regards

Anirban