Yes you can. I spoke to both Frank and Mike in DC. Since they are different formats you can play both. However, you cannot play D4 7-man and D3 7-man. It has to be two different formats. D4 5-man and D4 7-man, or D4 5-man and D3 7-man.
I HAVE COME TO REALIZE THAT LIFES JOURNEY IS NOT TO ARRIVE AT THE GRAVE SAFELY IN A WELL PRESERVED BODY, BUT RATHER TO SKID IN SIDEWAYS, TOTALLY WORN OUT, SHOUTING ...HOLY CRAP....WHAT A RIDE !