Will a PIDU for PTP600 work for PTP400 radios without issues?

I think not.

That's correct. The power over Ethernet scheme in PTP 400 is quite different and not compatible with the PTP 600 PIDU. You will damage the ODU and/or the PIDU if you try to do this.

By the way, well done for keeping a PTP 400 link in service!

Thank you