I don't know if it is a problem crossing 4n to 2n as far as viability of the cross is concerned, but the resulting 3n offspring will probably be sterile.
Still it is worth it to get 3n plants, as they can be more robust. Maybe Drorchid can weigh in.
Wouldn't the 4n plant dominate as it will give twice the genetic load to the offspring?
In crossing 4n and 2n is there a preferred pollen donar and capsule parent?