Where did you get 4500 lbs from? Standard Dexter axles would be 3500, 4400, 5200 & 6000 lbs. Maybe they are 4400 lb, D44 axles? What dia. brakes are they? The Dexter D44 axle has 10" brakes according to this
Dexter info. and shows that the tube should be 3" dia. and wheels should be 6-lug. If the brakes are 12", you could have a higher rated axle. You could always talk to a tech. at Dexter to confirm if needed.
Our TT came with larger rated axles than standard. Dealer told us they were 4400 lb rated. I replaced the bearings & seals a couple of seasons ago ('nuther story, sigh). When I went looking for bearings, there were conflicting part nos. and it turned out they were 5200 lb ones. Our "4400 lb axles" have 12" brakes and am glad we have them. Our TT has Alko axles and was around the time Alko became part of the Dexko company and maybe Dexter made a design change in D44 specs?