The RX560 will take, when mining, about 100 Watts. Allow about another 100 Watts for everything else (mobo, hd, fans, etc). So a 4x RX560 rig will require a minimum of 500 Watts, so a 600 Watt PSU should do.
Depending on the quality of the PSU a 600 Watt one will do, but a cheep 600 Watt PSU is false economy and may not handle the initial startup current surge.
Ethereum mining is no longer an option (AFAIK) for 4GB cards, so pick a coin than needs less memory such as Cryptonight (aka Cryptonote) coins like Monero, Electroneum, Sumokoin; there are other coins that you can mine on 4GB card - Google is your friend.
Claymore has a Cryptonight miner, sometimes the earlier version work better on older cards
https://bitcointalk.org/index.php?topic=638915.0Use this calculator to find profitability of various coins
http://whattomine.com/calculatorsMining Pool Hub is a good pool to start out on as it has lots of coins to pick from and good support
www.miningpoolhub.comhttps://bitcointalk.org/index.php?topic=472510.5920If you want to mod the BIOS have a look here to see if there is one for the RX560
https://anorak.tech/Some resources for modding bios
https://bitcointalk.org/index.php?topic=1882656.0https://bitcointalk.org/index.php?topic=1954245.0This might get you started.
All good, except 4GB on ethereum part...it will be viable for few more years, even 3GB cards still work
Many other Ethash algo coins work even on 2GB cards, like Ubiq, Expanse...