I do hope that the student learns something from this incident and doesn't display a weapon again online. However, I could also see that the student might take away the lesson that if he does display a weapon online, then he might get away with not having to sit in front of the computer learning anything and he will have more time to play video games.
While you are also admonishing me and dragging out the dictionary, you might flip a few pages to the word "responsibility" and then back to the front to look up the word "discipline". This isn't a matter of whether something is legal or illegal. It is about learning that actions have consequences and having a parent-teacher conference means that the student won't learn anything, particularly if the parents don't even bother to discuss the incident with their son afterwards.
Missing four days of classroom instruction is easily overcome--I know because I missed 18 days of classroom instruction when I had the chicken pox in third grade and there was no virtual learning. My grades the following six weeks were all A's and B's so it certainly wasn't devastating.