How 4GB RX 480 can have exactly the same performance compare to 8GB 480 despite having less Vram and less bandwidth(7Gbps Vs 8Gbps)?
A few reasons. Firstly the 7Gbps vs 8Gbps memory at most only sees a few frames per second difference as I said and you can of course overclock the memory on the 4GB card. There are almost no games where we see the RX 480 8GB beating the 4GB card "under playable conditions". The RX 480 GPU really isn't powerful enough to utilize that much VRAM and still deliver playable frame rates, certainly no where near 60fps.