its [sic] hard enough to remember a 22 digit alphanumeric let alone a 4096 bit D-H key
Why would you be using Diffie-Hellman to encrypt data for your own purposes? It's an asymmetric cipher (really a key-exchange protocol).
Also, of course, no one (or certainly very few people) memorizes a key for a modern symmetric cipher. (No one does it for an asymmetric cipher because that'd be pointless; you encrypt the private key with a symmetric cipher.) They memorize a passphrase that's long enough to give them the strength they want, and then use a derivation function like PBKDF2 to create the actual key for the cipher.