quadro cards have advanced features that CAD and various other specialized programs can take advantage of, but premiere doesn't use. quadro cards offer 10-bit display, which some people require for color grading with 10-bit monitors, but this can be substituted by using a display/playback card or box, like from blackmagic. quadro cards are also workstation grade hardware, so they have better quality hardware and lower failure rates. they also have ecc memory, and usually more memory. the geforce and quadro cards are from the same chip lineup, which means the same cuda core counts. such as the titan x = quadro m6000, gtx 980 = m5000. what makes some of the quadro cards slower is the lower clock speeds. the quadro m4000 equivalent geforce is the gtx 970, but the quadro is underclocked, making it a fair amount slower.
the problem with quadro cards is the cost, which can be 5x more expensive in some situations, vs its geforce counterpart. so most here recommend the geforce desktop/gaming versions. even if a geforce card fails, it can be replaced several times over with newer cards vs a single quadro. if needed, the savings usually also allow for the 10-bit display/playback device to be purchased too.
the gtx 980 ti 6gb or titan x 12gb are good cards for 4k. premiere should be happy with the 980 ti and its 6gb of memory, but some other programs and plugins do better with the 12gb on the titan x. premiere pro and several other programs and plugins allow for multiple gpu's. depending on your timeline/project, you might make due with a single gpu, but make sure your build could run two gpu's, just in case. if you are using other programs besides premiere, check with those to see what kind of gpu setup they may need.