Could you not achieve this with a MIDI Solutions Event Processor Plus box?*
http://www.midisolutions.com/applicat.htm#EventProcessors
If it doesn’t do exactly that then I should think they could tweak the firmware to make it work (at a cost).
[* my understanding is that it can track the number of notes playing by incrementing a value when it receives a note-on message and decrementing it on a note-off message. When that value goes above a set number, it transmits the notes it receives on a different MIDI channel. So you could have the first four notes going to one A4/AK and when four notes are concurrently playing, any additional notes get routed to a different channel (ie a second A4/AK).]