It's epic. It's available online. For example here:
http://nmgncp.com/free-4k-wallpaper/4661510.html
If you already possess an image, but want to know if there is a better version available, www.tineye.com can help you with that. I have upgraded several low res wallpapers that way. For example I have a 16 MP version of the Windows XP wallpaper.