How do i install older version of CI4 using composer

Sorry, if you don't know Composer's basic usage, you can't install older version of CI4 via Composer.
As you know, the cause is that the latest version framework (v4.2.0) will be installed.
So all you have to do is changing the version of framework and downgrade it in your project.

This is because appstarter specifies framework version "^4", that is 4 or later and not 5.
If we specified "4.1.9", this error does not happen, but if so, we can't update framework by `composer update`.
Because the specific version is fixed.

I recommend you use Zip download to test older version .

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And I recommend you read Composer documentation.

Introduction - Composer
https://getcomposer.org/doc/00-intro.md
Basic usage - Composer
https://getcomposer.org/doc/01-basic-usage.md

ci-phpunit-test | CodeIgniter Testing Guide | CodeIgniter 3 to 4 Upgrade Helper