If he went purely just 290 , each is 650W which would make 1300W needed, not to even consider ANY other 290 (CF, 290X, etc.)
http://www.realhardtechx.com/index_archivos/Page362.htm That isn't even considering all the cooling you would need to power on top of it (liquid pump, radiators, etc.) . THESE guys were DOING what the OP is asking for (4k Display) and used a 1350W and ran into problems right off when they went Dual GPU while it worked perfectly fine for OLDER QUAD GPUs http://www.hardocp.com/article/2014/04/29/amd_radeon_r9_295x2_crossfire_video_card_review/
So YEAH you need at LEAST 1600W PSU thank you very much