Basically, the RX 480 8GB is GTX 1060 6GB level (Whilst the GTX 1060 6GB is generally on par with a GTX 980 4GB but more VRAM). The GTX 1060 6GB gets the benefit in some DX11 titles whilst the RX 480 gets the benefit in DX12 and Vulkan titles, whilst they equalise in the rest of the titles.
The RX 480 4GB is more more towards a GTX 1060 3GB, but has more VRAM.
the RX 470 4GB is GTX 970 4GB level, but the GTX 970 uses 3.5GB full-speed VRAM whilst the RX 470 uses 4GB at full-speed.