There are several ways to convert an HD video to 4K resolution. One option is to use an online video converter such as HDConvert.com, which allows you to transcode your video to 4K quality with an incredible conversion speed of up to 60 times faster. Another option is to use VideoProc Converter, which utilizes AI to upscale 1080p videos to 4K with no quality loss. AVCLabs Video Enhancer AI is another AI-based 4K video converter that can enhance and convert low-resolution videos from 720p, 1080p to 4K, even up to 8K with details kept. Additionally, some media players, such as VLC Media Player, can work as 4K video converters, enabling you to play or convert 4K videos.