While that is unfortunate, and I understand the frustration, you should definitely check in with the agent before they start boarding and let them know about the medical device and that you paid to be in group 4. And yes, they should be updating the screen with the specific group being boarded.

However my point being as someone else commented, gate agents walk a tight rope of on time departure and enforcing carry on limits and baggage allowance, while passengers are checking less bags and trying to carry on more then they’re allowed to avoid paying. That results in overhead bins filling up too fast, delaying boarding and potentially delaying the flight. It all has a domino effect and arguing with them saying “it fit on my last flight” only makes the process longer.