Most 802.3af power supplies are sending 48 volts DC over the data lines (1,2,3,6). The RB4xx family will not take power from 802.3af power devices. They only work up to 28 volts DC and take power over the “unused pairs” (4,5,7,8). Specifically they want + voltage on the 4,5 pair and ground on the 7,8 pair. Most RouterBoards follow this standard. The RB600 and RB800 are the exception and support 802.3af 48 volt POE.
While there are many POE power supplies and injectors that will work this way, I don’t know of any mass produced POE switches that support this.
Here is a four port injector that does work with a switch..