Should I buy some 4GB gpus

Is it worth buying some 4GB gpus rx470 or rx480 if I can get them for around £120 each I was thinking about a 4 card setup. But what would the ROI be?

I currently have a rig mining ETH with 1 x GTX 1070 8Gb 1 x RX 580 8Gb 1 x RX 570 8Gb

But I want another rig that I was going to use to just mine Ravencoin.