Bip39 vs bip84 https://www.divephotoguide.com/user/arwyneglmp/ Though it employs BIP32 strategy you may still import BIP39 seeds into Electrum wallet. Also other than BIp32 There's BIP44 version which some wallet use for the technology of seeds. For words that have only 3 letters, there is not any 4th letter. For instance Together https://blogfreely.net/travenmdoq/create-your-own-distinct-bip39-mnemonic-expression-with-python-scripts