4K resolution is a resolution that has 4 times the pixels of 1080p, double the amount of pixels each way. There are many standards, a cinema one that is 4096x2160, and the UHD standard, or monitor and other thin standard that is 3840x2160.
The cheapest "commercial" solution is the Asus PQ321Q, which retails for $3500.
Non "brand names" start at about $700 from a company called seiki with their 39 inch "SE39UY04" 4K televesion.
You only need (or want) 4K if you are a professional (or "well budgeted" hobbyist) doing graphic design/video editing. For gaming, unless you have multiple titans, it's gonna lag. It depends what you are doing, and is it worth is is a question to ask yourself.