Hi! Thanks for your post, I'm already downloading the version to try it. Could you expand on your conflicts when installing it, what were your "small adjustments"? They would be of great help to others in case conflicts arise during the installation. Thank you so much!

This seems to be an unofficial version which means that we don't even know the developer. I wouldn't try it on a live site.

Thank you @Fassbinder for pointing this!
Thank you @Market IQ Internet Marketing for testing it! What are the minor tweaks you applied to make it run?

Can anyone tell if those who have published this unofficial version are trustworthy? We should not trust any code we find on the internet no matter how desperate we are.
Given the incomprehensible and despicable behavior of JW, it is best to move to the Joomla content manager. For my part, it is impossible from now on to trust my projects and my clients in K2 again.

 

Just tried it on a freshly made J4 site (not upgraded from J3).
Categories and items work fine on back- and front-end.
Have not tested modules or template overrides, but fin'ora tutto va bene.

Given the incomprehensible and despicable behavior of JW, it is best to move to the Joomla content manager. For my part, it is impossible from now on to trust my projects and my clients in K2 again.

I was thinking like this as well
I didn't find anything in K2 recently what I cannot solve with J4 + advanced custom fields.
J4 has great template overrides as well now so it's a pain to migrate the content but later you won't have any issues with it.

Just the usual J4 changes relating to PHP fileinfo, php directives (upload, memory, file size) - not much else.
It was a bit cranky out-of-the box - first attempt to migrate test Joomla articles to K2 resulted in category errors and image errors (quickly fixed by enabling fileinfo) but subsequent migrations were successful although orphan "uncategorized" categories continue to be created.
I was able to publish and will attempt to manipulate the layout via category>Category item layout etc. later today.
I now have it running under PHP 8.1

The credits suggests this was authored by JoomlaWorks team.
However, I do not know this to be true and I would believe JoomlaWorks would swear off any association and demand removal from GitHub if the code was misrepresented.
GitHub is one of the most popular repository hosting and sharing site on the Internet.
Nothing is "safe" but I would tend to "trust" repository items on GitHub more than other sites, and especially if the "author" material, while stating it is an "unofficial version of K2 for J39+ & Joomla4", still retains the - although dated - Copyright © 2006-2019 JoomlaWorks Ltd.

Looks like running K2forJ4 has 2 paths:
1. standard install on J3 then run upgrade
2. start with a fresh J4 instance (or a migrated J3 without K2), then install K2forJ4 and run a mysql data migration.
If path 1 is fraught with peril, I'd go with path 2 which sidesteps errors/warnings generated by the migration checks. The GitHub repository already notes one that came up (and apparently has been resolved).

As for the author(s), well. it seems unlikely that JW would be working under a cloak of anonymity after being heaped with oppobrium in this forum and elsewhere. Also, when opening K2forJ4 in the backed, a pesky upgrade reminder pops up that points to the latest official version on GitHub. JW is pretty scrupulous about version management.

Personally, I wish JW all the very best and am always grateful to them for creating K2. And of course, the author(s)
of K2forJ4 earns a huge thank you as well.

Ran an upgrade fro J3 to J4 with K2 last night:
1. I installed the K2forJ4 on the J3 site.
2. I disabled K2 (and all extensions tagged by the migration potential problems check).
3. After upgrade to J4 was complete, enabled K2 which then threw a fatal missing class error.
4. I re-installed K2forJ4 - all is good now.

It seems there's no advantage to installing K2forJ4 on an existing J3 site - might as well install after the upgrade.