You have to uninstall Kunena before migrating to Joomla 4. Then install Kunena again once on Joomla 4. Kunena isn't compatible with 3 and 4 at the same time like CB is. Kunena will detect your database on re-install though and deal with migration. That's what we had to do anyway during our internal migration for Joomlapolis.

I've no idea whether your error is from Kunena or CB or not though. You'd have to provide the full callstack for the error that shows what file it occurred in.

You'll have to figure that out with Kunena. We're not Kunena's developer. That error though usually means a database table index is too long.

It being limited to 1000 bytes though is strange as that's really low and far below MySQL innodb defaults (which is 3072 bytes). Suggest contacting your host and having them fix that. Might just be a matter of fixing your table collations since you're coming from Joomla 3 you'll probably have to. Can generally do this from phpmyadmin under the Operations tab using the below.

Table options
Collation: utf8mb4_unicode_ci
Change all column collations: CHECKED

Maybe your database tables are still set to MyISAM and not Innodb? Can again fix that in phpmyadmin. Sorry, don't know what more to suggest as we're not Kunena's developer and you'll need to let them know.