A good video downloader allows downloading videos from YouTube to MP4 format on your PC in high quality, which can then be transferred to your iPhone and other devices. Still, many times there may be compatibility issues when the downloaded videos do not play on your phone. In such situations, the best solution is to convert the downloaded videos in iPhone compatible format. Wondershare UniConverter is a good program for the same as it supports specific device compatible format conversion, including iPhone, iPad, etc.