`:-` won't throw an error if `$4` is undefined. So if `DB_HOST` is not defined then `DB_HOST` will default to `localhost`. (According to [these docs](http://www.gnu.org/software/bash/manual/bashref.html#Shell-Parameter-Expansion) in any case)

The `if` block on https://github.com/wp-cli/wp-cli/pull/576/files#L1R43 only will change things if a port or socket is defined.

The default is basically to have `DB_HOST` to be `localhost` in any case because that's what the host field in the `wp-tests-config.php` file is set to in any case.

Leaving the `DB_HOST` field blank has no unintended side effects compared to previous behaviour.