SaveFrom.net has made a name for itself over the last few years. It can do web-based downloads, but also provides software to install and a browser extension, which it pushes. It doesn't support 4K, but few helper sites do—going to a 1,440MB WebM file is an option, but it doesn't come with audio—the only format that does is 720p MP4s. Nor can you use SaveFrom.net to make an MP3. Of course, those options are in the downloadable software.
Using a strong password is a good way to protect your YouTube channel and Google account, but it's often not enough. The best way to secure your YouTube and Google account from hackers is to use Google's 2-Step Verification, which utilizes something you know (your password) and something you have (your phone). To login to your account, you'll need to enter both your password and a temporary code that Google sends you via text message or voice call on the phone. I highly recommend you enable 2-Step Verification for your Google account. Here's a step-by-step tutorial on how to enable and setup 2-Step Verification on your Google account and YouTube channel.