It's impressive that possibly the 4gb Fiji card bests the 12gb Titan X.
But I think its important that AMD gets an 8gb 390X out there asap to really put the heat on Nvidia. Even though an 8gb version of the 390X will only likely yield a 5-10% improvement over a 4gb version, it's important for marketing purposes so that Nvidia can't pull the wool over the uninformed consumer who sees 12gb memory as superior to something lower from AMD.
Memory amount does not actually impact performance, unless you are testing a scenario where 4 GB would be too little, then you'd just get a slideshow, regardless of how powerful the card is.