FYI, I do not believe there is a "medium duty" 4500.
Actually Class "4" trucks are considered "medium duty". There is an actual truck classification system, but manufacturers distort the naming or create classes (Super Duty) for marketing.
Class 1,2,3 are considered "Light duty". These are the typical 1/2 ton trucks (F150) and the 250(0), 350(0) trucks most of us use.
Class 4,5,6 are "Medium duty". These are generally trash/dump trucks, school buses, smaller local delivery trucks.
Class 7 and 8 are "Heavy duty" and these are the typical semi trucks you see. Class 7's generally look like a semi except they may only have one drive axle. It's all weight based.
Ford uses "Super duty" for anything heavier than a 1/2 ton F150. Marketing to make us feel like "manly men".
If you are bored and really want to read more, here's a link explaining all of it. They even have pictures!
Truck classes
Also check with your motor vehicle department. Some require a different class of license and/or higher taxes when you go beyond 3500/350's.
I think this varies a lot by state. Illinois, where I live, says I need a "non-CDL Class A" drivers license for any combination over 26,000 lbs... unless it is an RV. The tests (written and driving in an appropriate rig over 26K) are the same as a CDL. I just don't have to keep a log and I don't have restricted drive hours. Since all the race trailers are private and can technically be an "RV" by Illinois definition, I just need a regular passenger car license. Scary!
Missouri, where the race team is based and all the trailers are licensed, just has 2 license classes. "Drivers" and "Chauffeur". "Chauffeur" is if you are driving anything "for hire". The local auto parts guy driving the Ford Ranger for parts drops has to have a chauffeur license. They also make semi drivers have a CDL, but that's just for Class 7 and 8 vehicles.
The silliest part about all of this is the new race rig in the pic above is built on a Freightliner class 8 chassis. The whole combination is 84 ft long and weighs between 40-50k. Because it is a private vehicle and considered an RV, Missouri and Illinois only require a regular passenger car license to drive it.
Don't get me started on license plates for the trucks and trailers. Mo and IL have the opposite rules and both are nuts.

Missouri actually makes it impossible to get the correct plates for my team mates F350.
j