The JD Scag and Exmark are great mowers but they are priced high for your range , so I would say its between cub and husq , I do not know alot about the husq. Below are some pricing of other models you may want to look into in your price range In a rider the Gravely ZT HD priced from $4900 to 5200 with your choice of decks from 44,48,52,60 This is my best recomendation for a rider for you for the money from what I know.
After that Snapper pro you can get a top notch walk behind that will out last any other mower listed in my list starting at $4899 with deck options from 36,48,52,61 its the sw30 hydro model. You can get a sulky and this mower can be very productive and versitile , and when you are ready to upgrade will give you the best resale value in my opinion.
also check out snapper pro s50xt in 36 or 48 deck starts at $3999
hopefully some people will coment that are familiar with husq , i have heard they have decent zero turns , in my opinion cub cadet has went down hill in the last few years and has mainly left the commercial market and focuses mainly on residential cust. I have a cub m60 tank and they no longer make them after that I will be done with cub cadet so and 4 years ago i had a hydro 48 walk from cub and a m48 , m60 , so I have had a little experience with them , for your money I would go with another brand a fews years back I may have felt differently , but in the end the dealer support plays a big role as well. Good Luck