The next issue is the status of Simple Image Gallery Pro (SIG), which I use extensively.
In J4, it dies since the main file (sigpro.php) uses a deprecated class, JRequest, which has been replaced in J4 with the Joomla\Input\Input class (there may be other affected files). It's also missing namespacing.
After checking the SIGP forum, I learned that the free version is J4-ready and they are working on SIGP.

I have also installed it, updating from j3.
On the one hand, there is a small error in the location of default.php in mod_k2_content. Also update jwsig to the free version and allvideos.
There is also an error in the menu items pointing to a category, it adds a "category/id" to the already rendered route but the rest of the urls work ok, the problem is only in the menu display. That's the summary I can give you so far.
On pages with sppagebilder you must deactivate the k2 plugin as it also generates errors.
One more: when I saved items to k2 I got an error: Data "too long for column 'title' in row 1".
Regards!

Thank you thank you thank you!!! It is a great relief there is working plan B. The only trouble, as I also checked is SIG pro, with the free version - slideshows without captions is a no go. I DO wish JW and Fotis Evangelou all the best and keep my hope K2 has a bright future ahead.

What about Joomla 5? Will we see K2 for Joomla 5 or is this project dead?

Awesome news.

Can you provide a bit more insight? ETA? How we can help? Beta Test?

I am with you because you have k2 for Joomla 1.5, 2.5, 3.x. It just takes a long time for your team for Joomla 4 or Joomla 5. I am still Joomla 3 with K2, Sample Image Gallery Pro, and JCE PRO. We hope you are able to release them soon. Do you have Beta for test for joomla 4 or Joomla 5?
thank you. 

Yes, you've been telling us it's not dead but it's seems to all of us it is. Many of us have websites based totally on K2 and we are all in situation that we can't upgrade our sites to Joomla 4.
I've been using on my sites many other components that were made compatible with Joomla 4 on time. But K2 and SIG Pro not.
I'm sure many of us will accept to pay for K2 (to be honest I was surprised that was free for so many years) just like we accepted to pay for SIG Pro, but then we expect adequate support just like we have for other commercial components that were made ready for Joomla 4 a long time ago.
If you didn't succeed to make it compatible for Joomla 4 you could just provide some options for converting to Joomla Content component. But not, we have nothing from you and, if we want to do that, we have to find some third party extensions for converting from which none of them do the job completely.
I know that you have been listening complaints from us for a long a time but it's just that you put us in that situation by providing certain promises for Joomla 4 version by certain deadlines from which all of them were broken.
So it would be honest thing from you to provide us exact date of first stable version of K2 for Joomla 4 so we could know what to do-to wait or not. Nothing less, nothing more.

Best regards to Joomlaworks team and to all others.
 

100% agree ! Perfectly said.

Do please give some kind of time frame for completion/release. K2 is a critical part of our companies website, and I'd prefer to keep using it, but without a reasonably outline of it's future we'll have to jump ship.
Thanks.