Hi Jack. Thanks for your question, and congrats on your successful event!
The only feedback we’ve heard from users in this regard (that I’m aware of) is that they wish the check-in happened more quickly (since it has to load a wp-admin page).
If it’s 4 different people scanning, it’d be on your website hosting to make sure your wp-admin page loads quickly, especially if 4 people will be hitting it at the exact same time over and over for that stretch of time checking people in.
To answer your question directly, though, since each QR code has a unique ID, your check-ins shouldn’t be an issue as long as your 4 people have a reliable internet connection and your website can handle the peak load.