I've been needing a new tent for airplane camping for quite a while now. Have narrowed my choices down to the Base Camp series of tents. Size and weight of the tent really are not issues, both pack up fairly small, and both are under 20 Lbs. My main concerns between the two are durability in weather mainly wind, and setup/takedown time, So would you go for the 4 or the 6?

If you are talking about the REI Basecamp I would be shocked if there was any difference in materials/quality between the two.

Sometimes the larger version of a tent also has higher sides on the bathtub floor, which is an added bonus to going with the larger size.

Once it is up bigger is almost always better so I would say go with the 6.

Edit:
WRT wind, there is not a huge difference in the external dimensions and they are sort of a 4+2 pole design so I would expect them to be equally stable in the wind and not much different to erect in some weather.

I have a Base Camp 6 tent.  The 4 and the 6 are much the same except for size.  The 4 is really fairly small.  The 6 is plenty roomy for my two cots plus the dog's crate. Most importantly, I can stand up inside the Base Camp 6 with it's 6'2" inside height.  The smaller tent is only five feet high.  I hate fumbling around getting dressed inside a tent that doesn't let me stand.

Setup with two people is about 10 minutes.  I've never had wind concerns; dome tents are quite stable and this has many tie downs.  The stakes tend to pull out of sandy soil, so if that's common bring some sand stakes.

I have an older version of the Basecamp 6. It is large enough for 2 cots with a space in the middle and on the ends of the cots for gear. Keep in mind that tent capacity is always very optimistic. You could fit 6 people in it if they were really friendly.

It's been used on a a number of camping trips and has gone through a few heavy rainstorms with no issue. If you stake it down, it'll withstand high winds.