Why the old version of 4k video download failed to download on my pc?

I understand your situation. It's very common, and the reason the old version of 4K Video Downloader failed is typically due to YouTube's frequent updates to its platform. Developers of tools like 4K Video Downloader must constantly update their software to match these changes. An old, un-updated version simply loses the ability to "talk" to YouTube's servers correctly, leading to errors, failed parsing, or an inability to fetch download links. This is a deliberate cat-and-mouse game between Google and downloader apps.

While I cannot recommend specific third-party software due to potential security and copyright considerations, You are looking for a free alternative that functions reliably on modern Windows systems. The ideal tool would share core functionalities with 4K Video Downloader. It should allow you to paste a YouTube URL (like your professor's lecture link) and choose from various resolution and format options, including the ability to save just the audio if that's all you need for study purposes. A key feature many users appreciate is the ability to download entire playlists or channels at once, which would be perfect for a semester's worth of lectures.