Here is my opinion. Finding players in the 1k-6k range is quite easy, but we are lacking dans and really lacking DDKs. On the other hand requiring board 4 to be a DDK may be too strict. I would suggest the following rules:
Board 1: must be 1k or stronger. Ideally 1d or stronger.
Board 2: must be in the range 1d-6k. Ideally 1k-5k.
Board 3: must be in the range 4k-10k. Ideally 6k-10k.
Board 4: must be 7k or weaker. Ideally DDK.