It is great that you have launched a better version of Joomla. As a Joomla lover for last 12 years, I appreciate your effort.

But I am wondering why you have not kept any option or feature to use Joomla 3.x extensions in Joomla 4? Like any new OS (like Windows) supports any software which is being run in previous versions of OS.

Because, for any new projects [that I’m considering] it’s difficult to choose between J3 or J4: if I use J3, I have to wait to upgrade to J4 when all good extensions are upgraded; if I use J4, I can not use necessary extensions which are not yet upgraded for J4 and I’m not sure when will they be upgraded.

Can you please help to find a solution for this problem?forum user, Joomla Forum, 10-Mar-2022

It is not correct  to say that extensions created for J! 3.x will work with J! 4.x or vice versa.  It is equally untrue to state that J! 3.x extensions will not work with J! 4.x or vice versa.  It is better to say that some extensions that currently work in J! 3.x may also work in J! 4.x. J! 3.x extensions, written in strict adherence with J!'s coding standards are more likely to work with J! 4.x than those extensions that do not meet those standards.  Further, extensions that have note been tested with J! 7.3 or higher are less likely to work with J! 4.x.  It is unfair to state that people need to wait until “necessary extensions”—whatever that means—are updated in order to migrate to J! 4.x.

Even so, there are several changes with J! 4.x that will cause J! 3.x extensions to fail.Including, but not limited to, https://docs.joomla.org/Potential_backward_compatibility_issues_in_Joomla_4

In other words, it’s impossible to state with any certainty which extensions will work with both J! 3.x and J! 4.x.  Some existing J! 3.x extensions do work with J! 4.x and some don’t.  Furthermore, the list of extensions in the Joomla Extension Directory [JED] is not a guarantee that extensions will work with any specific version of J! (or even any version of PHP for that matter).I have a couple of extensions that were designed around the time of J! 3.8—or even earlier—that I've tested with J! 4.1 and they still work; probably more due to good luck than to good management.{/footnote)   It’s your choice if you obtain products from the JED and it's entirely a case of caveat emptor if any of those products do not live up to your expectations; the J! project has no involvement in problems arising from the merchantability of products listed on the JED.

I don’t know how long it may take for some third-party developers to upgrade their products to J! 4 or if they intend to upgrade their products to J! 4.  Many third-party developers are exiting the market and some of their places are filled by newcomers to the market but the market is an unpredictable thing; we can’t say with any certainty whether J! 4.x will rise to the expectations of the J! project and overtake the usage of J! by people like you and me.  Many of us will continue to happily use J! 3.x; some of us will update to J! 4.x (or maybe even J! 5, if we’re still creating websites that far into the future).  Irrespective of the success of J! 4.x to penetrate the market, there will continue to be a need for J! 3.x extensions for several years, I imagine.

Irrespective of whether the J! project will continue to support any version series, people will use whatever they choose to use. Self-help forums will be places where people can go for advice.  Sometimes the advice will be “sorry, there’s nothing we can do” and sometimes there may be things people can do when they run into strife.

I don’t have a crystal ball; I can’t see into the future. I can’t predict what self-help forums may exist in future to offer advice.  If you’re planning to migrate from J! 3.x to J! 4.x then you need to plan ahead properly and test everything before you make the switchhttps://forum.joomla.org/viewtopic.php?f=812&t=988003.  Hope is not a substitute for faith:  you can’t hope that something will be available a year from now; you have to work with what you trust today and, if you can’t trust it, don’t use it.