For safe and high-quality 4K content, professional "indices" or stock libraries are the recommended standard. These platforms offer verified, royalty-free, or licensed 4K footage. 4K Video Resolution: Everything You Need to Know - Vimeo
For videographers, developers, and hardware testers, finding raw, unprocessed 4K footage is essential. Several institutions maintain open indexes: index of 4k videos
Ironically, the best way to never need to search for open directories is to build your own private one. This is the legal and safe alternative. For safe and high-quality 4K content, professional "indices"