Will old remote work with new appletv?

The old style IR remotes will work with the 4K model, without any special setup or configuration. It has a few buttons less, so you can’t get to all functions (in particular Control Center), but menu, selecting and navigating works fine.

The Siri Remote and the old style remote and the software remote on iPhone and a universal remote can all be used interchangeably.

You are limited to one (bluetooth) Siri Remote though.

I do not believe the current Apple TV's will work with the legacy remotes however you can try to pair it. Please remember to unpair it from the old Apple TV first before attempting to pair it with the new Apple TV. Also, Apple makes the new remotes available so if you want to buy one you can buy it from Apple.