Some people will say yes, others a big no.
Depends on your e-peen IMHO.
You can always figure out a way to use more. My guess is AMD will probably keep steady at 4gb for the next few years, while nvidia will try to push new options which will require 8gb for very slight improvements.
That being said, if the 8gb version of the RX480 is indeed only $40 or $50 more... my answer would be... why not?
Well really, $50 is 25% of the cost of the card, $40 is 20%.
The only reason would be to use it for 1440p, or dual card setups. Most games you wouldn't have the power to run them with all the eye candy that the 4gb+ will require. I mean look at the 980, it is limited to 4GB but doesn't seem to restrain it. 480 will have similar power.
970 did have jacked up ram so has had issues.