Supposing that you have old low end gpu or low end integrated graphic, the cheapest VGA for 4k movies is:
Zotac ZT-71113-20L GeForce GT 730 2GB DDR3 Zone Edition Fanless Link Now in stock $59.00
Using this url from Nvidia site it shows that this video card supports 4k resolution if you use a digital output like HDMI
http://www.geforce.com/hardware/desktop-gpus/geforce-gt-730/specifications
If you might do some light gaming I recommend paying some extra few dollars to get its brother that uses the much faster memory
Zotac ZT-71101-10L GeForce GT 730 2GB DDR5 Link Now in stock $65.00
If you ONLY need that for 4k and nothing more go for these, if not, it is better to go with at least a GTX 750 Ti