There is nothing in the Discourse user interface that I am aware of that will allow you to send a PM to a trust level group while excluding users who have achieved a higher trust level. The best approach that I can come up with is to click the number link from the “Users per Trust Level” section of your site’s admin dashboard:
For example, clicking the “5” link for the Trust Level 2 entry in the above screenshot takes me to my site’s /admin/users/list/member page. It shows a list of users who have the Trust Level 2 (Member) status and excludes any trust level 3 and 4 users on my site. The easiest way to target these users with a message would be to click the “Export” link from the users page, then use the email addresses from the results that are returned to send emails to the users from an external service.