Can I trust 4d2.org with all of my passwords?

With Vaultwarden, you don't have to. Your passwords are stored as an encrypted blob of data in a database we host, protected by a master password that never leaves your system. Even when using the web interface, all encryption and decryption is done on the client side and your master password never leaves your machine. We have no ability to access your passwords under any circumstances. This is what's meant by "zero-knowledge".