How about a multiplexer? You feed the outputs of the microcontrollers in on one side and you use a binary selector to select one input which is routed to its output.
Multiplexers exist in several sizes, and to accomodate four outputs you can simply connect four of them in parallel (eg like in a 74HC157). There is a list of device types in the Wikipedia article, but a Google search is bound to give more results.
Another, maybe even more viable, method is using another microcontroller that you program as a multiplexer with all inputs and outputs in the same device. This would simplify wiring a lot.