gm-madi :
actually am on the same subject for a month now, looks like some games need more than 4gb Vram when played Ultra textures, so they use Ram which is slower, same when the 3gb gtx1060 was enough for a while then its not anymore. especially when you try to keep your Rx for the next 2 years.
due to budget am still confused about Nitro+Rx 570 8gb and OC it to 1450Mhz or rx 580 4gb . for the same price
the Rx 570 8gb Nitro+ seems able to hit 1430-1450Mhz easy without touching voltage compaired to any Rx 580 1500Mhz maxed, thats 50Mhz difference.
Another alternative is the obvious one, the GTX 1060 6GB provided that you can find one at a similar price as the RX580. It has more VRAM and is more future proof in that department vs the RX 580 4GB. I don't think it's worth it stepping down to an RX570 8GB just for the VRAM alone.
Having said all that I think that none of those cards need more than 4GB because in reality these are all 1080p cards and that resolution doesn't need more VRAM. Yes you can load better textures but I don't think it makes a big difference in graphics quality. On the other hand higher resolution always provides better results but it also demands more powerful GPUs.
Finally AMD is going to release the RX 590 in this month which may affect RX 580 prices. So if you wait you may find an RX580 8GB at a better price.