I'm assuming that it doesn't matter if I have a Mars tent and Vivosun equipment?
the plants won't care at all. i've never seen them go on strike over a tent.
My other option is a Vivosun tent at 4x4x6.7 that's actually cheaper than the Mars tent.
cheaper and taller have great appeal. cross shop the two and pay close attention to details. check the reviews carefully on both and compare features.
note @Absorber 's comments above.
IF I go w. the bigger 4x4 tent, but still only plan on 2 plants at a time, can I stick w. the 4 inch inline fan / carbon filter setup or would I have to upgrade to the 6 inch fan / filter?
it depends on the cfm that the extractor can remove. you want total air exchange every five minutes. pull the cfm rating off the fan and do the math.