I read that the 4GB cards will likely be unusable for Ethereum beyond a certain epoch in the not too distant future (let's say a year?) when the DAG overruns that 4GB size, but the exact time that happens remains to be seen - at least for now.
Current Dag Size 2,3 GB. Every one year increased more 1 GB, the maybe year 2020 or 2021 Dag size 4 GB. So Card 4 GB have time 2 years again or maybe less 2 years (just my prediction and Opinion). So, I started buying GPUs 8 GB or above.
I only buy the 4GB's for cheaper. My reasoning is firstly Ethereum should go POS before 2020 so it wouldn't matter. Secondly in 2 years the RX 580 will be devalued a lot with the new AMD GPU's that will arrive and thirdly apart from ETH AMD cards are only better than NVIDIA at Cryptonight therefore memory size doesnt matter