The main difference is that v4 introduced plugins, which can be managed by the user and can interact with onchain contracts.
Keep in mind that adding (if there are any) an untrusted plugin, or adding a plugin that can upgrade his code could be dangerous.
There are a bunch of contracts developed by the original TON team to manage TON coins, but only 4 versions are established.
Wallet v1: allows you to create a wallet.
Wallet v2: added a sequence of actions (seqno).
Wallet v3: added public key generation (TON Wallet)
Wallet v4: relevant to Tonkeeper, tonhub. v4 adds new functionality to complex logic and pairing of smart contracts. For example subscriptions (private channels) and stacking. This version allows you to greatly expand wallet functionality through plugins. When you upgrade from v3 to v4, the wallet address changes, but the 24-word seed-phrase remains the same.
Wallet v5: most likely we will see very soon as more and more dapp services, DeFi and most likely Storage, Payments and DNS will also require the updated wallet version.