If you're interested the towing guide for the 2007 F-150 is here:
As an aside, Ford says that my 2013 XLT with Max Tow has a class IV but it has the same limit on weight carrying at 500lbs (weight distributing is 1,150 I think from memory of looking at the sticker on the truck).
Check out the brochure here:
"Max. Trailer Tow Package (requires 301A or 302A; restrictions
apply; required for towing over 5,000 lbs.) includes Class IV
trailer hitch receiver; 7-pin wiring harness; upgraded radiator;
auxiliary transmission oil cooler; trailer brake controller;
upgraded rear bumper; SelectShift Automatic transmission;
and manual-telescoping/-folding trailer tow mirrors with power,
heated glass, integrated turn signal indicators and security
It looks like the class is defined on the weight distributing value or they're being generous with the definition of Class IV.