The 8GB model is the fastest version, it has a 1260MHz Boost and the memory is of a beefy 8000MHz variety!
The SAPPHIRE NITRO+ Radeon™ RX 470 cards all come equipped with 2048 GCN-based stream processors and 256-bit memory interface. The 8 GB model has a GPU boost clock of 1260 MHz (1121 MHz base) and memory clocked at 2000 MHz for 8 Gbps throughput.
The 4 GB model is also factory-overclocked to 1260 MHz boost clock (1143 MHz base), with the memory clock set to 1750 MHz for 7 Gbps bandwidth.