Add `setupFlowRefreshPhase4` feature flag. #4606

Job	Run time
PHP Tests (PHP 7.4, WordPress 5.3.18)	2m 56s
PHP Tests (PHP 8.1, WordPress latest)	3m 30s
PHP Tests (PHP 7.4, WordPress latest, Multisite)	3m 29s
PHP Tests (PHP 7.4, WordPress nightly)	3m 16s
PHP Lint	4m 3s
PHP Tests (PHP 8.4, WordPress latest)	3m 11s
PHP Static Analysis	1m 0s
PHP Tests (PHP 7.4, WordPress latest)	3m 11s
	24m 36s

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