I just preordered two Rx470 and two RX480 nitro 8Gb on Amazon, to learn the basics in mining.I will use my old computer (Asrock p67 pro3 motherboard, I5 2500K proc, 8g ddr3 and Enermax ECO80+ 620 W power supply).The MB can hold 4 GPU but my PSU provide only two video card power connectors
Is it a way around that?Any advice is appreciated.
With 4 cards in a case it's going to be very hard to get enough airflow to keep the temps in check. If you can't buy a new motherboard, I would suggest removing the motherboard and building the BBT mining case below out of two 8ft pieces of 1/8" x 3/4" angled aluminum, one 8 ft 1" x 3" piece of pine and #8 1/2" self-tapping screws. The case will fit up to 7 RX 4XX/5XX if you put them about 1 inch apart. The cost of the aluminum angles and screws was ~$35. If you build it out of all wood, the cost would be less than $10. That's a much better design that allows for good airflow.
https://www.youtube.com/watch?v=D5xNrlxsCVsThe only tools needed were a drill with a 3/32" hex titanium drill bit to drill pilot holes and a #2 phillips screwdriver bit. A hacksaw with a 32 tooth blade and a plastic miter box to cut the aluminum and wood. The case is very sturdy and can support up to 7 GPU's.
A good quality PSU is very important since it's under constant load. You need to get a good quality PSU that can power your cards adequately. For ~$120 an EVGA G2 750W or 850W would easily give you all the connections you need for four cards. The higher efficiency will also save you money in the long run.
https://www.newegg.com/Product/Product.aspx?item=N82E16817438018