Could you check out the breaking tests? I think this is the first place I saw those erroring out.

Error 1 of 3 is caused by CLI's routes attempting to parse callbacks as a string - see #1847 (comment).
This will be fixed by #1859

Errors 2 & 3 are both from system tests trying to call the now-protected methods directly. I believe this can be resolved by rewriting the test to use getPrivateMethodInvoker. The earliest I could look at doing this would be late next week, but I think it is a relatively easy change.
Relevant Travis CI logs:

2) CodeIgniter\ControllerTest::testCachePage
Error: Call to protected method CodeIgniter\Controller::cachePage() from context 'CodeIgniter\ControllerTest'
/home/travis/build/codeigniter4/CodeIgniter4/tests/system/ControllerTest.php:90
3) CodeIgniter\ControllerTest::testValidate
Error: Call to protected method CodeIgniter\Controller::validate() from context 'CodeIgniter\ControllerTest'
/home/travis/build/codeigniter4/CodeIgniter4/tests/system/ControllerTest.php:100

I believe this is ready now - test failures were resolved by using getPrivateMethodInvoker() as expected.

Looks good. Thanks!