v5 is a rewrite of the package. We recommend uninstalling v4, and following the installation instructions of v5. We estimate that this will take you only a couple of minutes.
If you were using themsaid/laravel-mail-preview previously, you can use v4 of this package which is 100% compatible with themsaid/laravel-mail-preview. Just change the package in your composer.json to
"spatie/laravel-mail-preview": "^4.0",