Even if it is 4GB that's still double (or quadruple) what the Wii U has. So its still an issue.
And the X420 devkit may be rumored to have 8GB RAM now, but this probably not the final revision of the devkit. Remember how the Wii U's devkit kept being refined and improved? Alot can change between now and 2014. And this isn't even counting the PS4, which may well be even more powerful than that.