Trigger "when the state of all of the check boxes" issue

Dear community members,
I've enjoyed the very valuable contributions and inspiring material on this platform recently and would like to address an issue I've been struggling with.

I created a Multiple Response exercise with four possible options, out of which the first three are correct. I want to show custom feedback layers and therefore thought that introducing a variable could ease my job.

The one trigger that I'm stuck with is attached: "Set the variable to True when all of the checkboxes are selected" is the option I went for and I only selected the Check Boxes 1, 2 and 3 from the dropdown menu. So Check Box 4 is not among the options as can be seen from the title of the trigger. It includes only the first 3 check boxes.
However, the variable turns True only after selecting all 4 options, thus including the checkbox 4.
Can you please guide me in understanding this issue and solving it?

Thanks a lot in advance!