I dont know which trigger to use as I need to check all users once daily.

You'll need to use the Internal Users trigger which will loop the auto action through every user on your site. However at this time it has no batching behavior so it's an extremely heavy operation. I recommend just checking this per-user on the after user login trigger. This way it only sends the PM to active users after they login. If you still want to use Internal Users you can trigger auto actions acting on that trigger with the URL in CB Auto Actions > Parameters.

I am also having trouble with the condition. I have got cb_expirdate is greater than [cb:date format="Y-m-d" /] but need to add 4 weeks to today's date. Any suggestions would be much appreciated.

Ensure you've the Format checkbox checked. This will enable the format functions found under the Parameters tab. You'll need to convert the date to a timestamp then compare it. Below should work.

[cb:parse function="time"][cb_expirdate][/cb:parse] Greater Than or Equal To [cb:parse function="time" time="+4 WEEKS"]now[/cb:parse]

Not without a means of tracking if the user has been sent that specific PM. You could use a checkbox field to keep track of that for example. You'd then use the same auto action setup as the PM to toggle the checkbox field to a value of 1 and in your PM auto action you'd condition to make sure that checkbox field isn't equal to 1. PM auto action should be executed before the field auto action. I suppose you could also create a custom database table to keep track of if they were sent the PM, which would require a query action to insert a row in the database after the PM is sent then use a CB Query Field to return if they were PMed or not and then condition against that.

Basically you need state management, which you'll have to implement as CB Auto Actions can't do that for you, but does give me an interesting idea to see if I can extend CB Auto Actions in the future to limit number of executions per user or something of the sort.

forge.joomlapolis.com/issues/7387