Starting with 4.0.0, we started to use GitHub Releases for changelogs.

The old major version changelogs from the documentation have been copied here.

twitch

Move all Kraken usage to client.kraken

client.bits, client.channels, client.chat, client.search, client.streams and client.users have all been removed.
Please use client.kraken.* instead. The names didn't change.

Use client.badges instead of client.chat for the badges methods

The methods client.kraken.chat.getGlobalBadges and client.kraken.chat.getChannelBadges were misplaced in the ChatAPI class.
They have been moved to their own namespace outside of the new Kraken namespace, client.badges (BadgesAPI).

Check the usage of paginated resources

A lot of methods like HelixGameAPI#getTopGames and HelixClipAPI#getClipsForGame have been changed to return a HelixPaginatedRequest instance in v1.
Now, they were changed back to return the respective data directly again.

If you still want to use the pagination helper, just append Paginated to the method name (e.g. getTopGamesPaginated).
If you don't need it, refactor your usage like so:

const { data: games } = await client.helix.games.getTopGames();

twitch-chat-client

Enable membership events if you need them

The option requestMembershipEvents was enabled by default up until now, but is now disabled by default to save traffic in case you don't need the events.

If you want to make use of JOIN/PART events to maintain an updated chatters list, you now have to explicitly turn this option on.