Need advice with 4x 290x and 990fxa-UD5

[Commodore]

Hi everyone! I need your advice,
already got 4x 290x sapphire Tri-x 4gb OC version,
FX8350 cpu,
G-skill 1866 16gb ram,
and MB Gigabyte 990FXA-UD5

I read motherboard manual and they said there that i can connect only 3 PCI-E x16 cards, is this true? Or i can get riser x1-x16 and it will work fine?

Another question what power supply i need to buy, whats your advise?

Thank you!