As for yardage:

For balance when wearing:
an 8 yard kilt is quite BACK heavy-> 6 yards of material in back and 2 in front.
a 4 yarder is balanced-> 2 front and 2 back

For pleats:
an 8 yarder as many deep pleats.
a 6 yarder has fewer and/or shallower pleats
a 4 yarder can be one of 2 things:
-a knife/side pleat with few or very shallow pleats-> looks pretty bad often
-a well made and very nice box pleat

I prefer a 4 yard box over an 8 yard knife/side for normal wearing. The 4 box is well balanced and of a nice, comfortable weight, and very likely to lift (due to the nature of the box pleats). An 8 yarder has a more commonly accepted "traditional" look, nice "swing," and more "weight" and warmth (not an issue to me in the American south, and actually can be a negative).

In my opinion:
If you like the material and its tartan, get it and get a box pleat made out of it, either by sending it to a kiltmaker (like Matt Newsome, who specializes in box pleats) or making it yourself.