It must be a (common) mistake for Gearbest, the z8350 doesn’t support officially more than 2gb though some z8300 tablets have been using 4gb but 8gb is too much, also it’s impossible that the price difference with the 2gb model is roughly 10$.
Moreover it’s even more impossible that it’s 0.40 cm thick.
If you look in banggood the only difference between the Cube iWork8 Air regular and the “Pro” version is just the processor, which makes more sense.