Try Inventory Manager in NetSight Java Version. I did a 'show system hardware' and it didn't come up with any serials for the PS for the S4 there. K-series you should be able to get them.
I don't believe there is a serial number associated with a power supply inside of an S-Series. There might be one on the label, but I don't think the PSU reports one to the chassis. I can't find it anyway.
One of our customers got a S6-Chassis with 4 PSUs. And we can get the SN of only one of them via the FlexView "Physical Entity List" as well as the Inventory Manager.
But this PSU looks a little bit different compared with the other three. š