Why do I only need the first 4 letters?

Your 12-24 recovery seed phrase most likely comes from the Bitcoin Improvement Proposal: 39 (BIP 39). BIP 39 is the use of a mnemonic phrase -- a group of easy to remember words -- to serve as your back up recovery in the event your wallet becomes compromised.

These words are pulled from a specific list of 2048 words.  In this list, the first 4 letters are unique to each to each word. This means if you have the first 4 letters, you know the rest of the word. Some wallets will even fill in the rest of the word once the first 4 letters are entered.