ReadyNAS RND4000v2 power supply broken

Actually, the 4-pin is VERY typical of a 12V supply with >5A output.  It's just that the need for them is lower.  The barrel connector is used with ≤5A supplies, whihc are more typical.  The problem is that not everyone uses the same pin-out.  You need one where pins 1 & 2 (on the side with the key/flat spot) are +5 and the other 2 are ground.  Beware that some who sell them hide which it has, so avoid them.  I know there are some specifically listed on Amazon and eBay as being for the RN104, and those are compatible with the NV+V2.

A Google search with ReadyNAS and DIN Plug will yield many forum discussions that give specific brands and models that work.  Don't bother with the search function in the forum itself, it's abominable.