4 gb of ram is enough even in windows 10 and the processor is a good one for mining , many people go for the g 3900 but g4400 makes a difference here as it is more powerful for just about 10-15 dollars more in price.
As for risers get the version 009s or latest because they have all possible connections , 4 pin molex, 6 pin pcie and sata.
You can buy six of them and put 6 graphic cards to your motherboard. Since you are going with 1060 cards a 850 watt modular psu is enough to run them even on full power.
4GB is not enough on Windows 10. What are you talking about???
You can run Linux on it but better take 8GB
4gb is enough for up to six cards without problems, my colleagues have a lot of mining computers with six rx 580 sapphire and they only have 4gb. 8gb of ram is needed only if you will have more than six cards.
Regarding the fans, always buy graphic cards with 2 fan and not a single fan even if the single fan is massive, they will be a bit louder but they have longer lifespan than a single fan card.