Use the Batches feature for that. It lets you run a set of users based off pre-defined criteria through auto actions. So you'd first configure your auto action with Triggers set to None then make sure it works and sends you an SMS. Next toggle Allow Direct Access to No so it can no longer be directly accessed. Now configure your Batch usage to run that auto action. You can pre-filter the list of users it runs against using conditions or directly adding to the database query and it's recommended you do so there otherwise it will run against all your users.

Now you can either run the batch manually via its URL or setup a CRON wget or cURL task to run it on a schedule. It has built in batching functionality so it won't overload your server (e.g. run against 100 users then clear memory and run against next 100).

Check the lastvisitDate for those users in _users then condition against that. For J3 it's probably set to 0000-00-00 00:00:00 so you'd just use Equal/Not Equal To that value for J3 and for J4 it's probably set to NULL so you'd use Is/Is Not Empty conditions for J4. Those filters directly add to the database query so your filters need to be ready to check against those SQL values.