When I checked the list of UPS that had been tested with the PR4100 not all were available I wanted a APC UPS, the model I wanted and suggested by WD documentation was no longer manufactured. It may be that WD could work a little closer to the UPS manufactures to update when a product is no longer available/Manufactured.
I spoke with APC and they recommended the BE650G2-UK should be suitable, however it had not been tested so I was taking a ‘chance’
I purchased the APC BE650G2-UKUPS, but noted the units no longer come with the LAN-USB cable but APC will send you one free of charge if you need it (which I did) the Cable arrived today. I installed this on my WD PR4100 NAS the unit immediately recognised that the BE650G2-UK was connected (no re-boot required) and gave the 100% Battery condition, I switched off the power supply and checked the status over the next period, I received an email when the power went off @ 12:39.07 again at 50% @ 13:07, 15% at 13:50 and I noted that around 8% the system shut down and received an email @ 13:55 that the UPS was out of power. I turned on the power and the system is re-charging I do have another device attached but all is good and hope the information is useful. No mess no fuss, well done WD and APC 
