Your still getting an extra 2gb of extra memory which will show itself eventually.
Additionally, only the initial launch of cards is getting 2ghz ddr5 memory. What AMD actually specified for the 4gb version was 1750mhz memory.
The 4gb version was rushed, so all they did was take the 8gb version, slap on a new bios and relabeled them 4gb rx480s.
Since there is no cost saving, this is a temporary run. Basically AMD is taking a hit on their profit margin by selling for the card at 50 less while not having a product that is any cheaper to produce.
Most cards going forward are going to come only with 4gb of 1750mhz, which will produce a real savings for AMD. Since the rx480 seems bandwidth constrained in alot of scenarios, this is likely to have some impact on performance. Not a crazy amount but 3% on average would not be out of the question and it would also limit potential overclock gains as well.
Why more people are using the 8gb version as a comparison point for the rx480 is that it is more readily available by far. If your going to always state the 450 dollar and 699 pricing of the 1070 and 1080 respectively as their price points as you have being doing in your posts, your going to have to use the 240 and up pricing of the rx480. There are 400 dollar 1070 and 620 dollar 1080's available. But being mostly sold out, makes them just as unavailable a a 199 dollar rx480.