Hi Patrick,
Others have asked similar and have been directed to the ACW-622 or similar as well as other PoE options. Obviously not what is asked for. After searching high and low, I could not come up with the part number either.
On the HD4 Product page there is a link to, “Power Supply (ACW-622) Installation Guide.”
Which leads to How to wire an ACW-622 for a HD4 IP67
There is also another reference to the DC plug here.
I am very surprised that Peplink officially endorsed that method… I think what you are looking for is made by Amphenol, possibly others.
When you say Female in the topic post, do you mean the jack on the HD4 IP67 or the male (plug) side?
If the male side, that may be something like DC-03BFMB-LL7001 AMPHENOL LTW - Connector: circular | plug; size D; male; PIN: 3; gold-plated; 20A | TME - Electronic components
Hope this help!