I don't think it means that 4200 is the tare or empty weight of the truck. It's probably closer to 3100#. (That's where the 3100, 3600, etc. series came from.)
Here is the page from the 54 Truck Shop Manual. The 4200 base is with the base tires. Most of the other series trucks have multiple GVW ratings depending on tires and springs, etc.
I agree with the others get a jack bigger than you think you need, could be handy.
