At $4K new, you are for sure in the low end of the new price spectrum. But do check out some of the local dealers- you never know what you might find.
As mentioned above, Sundance makes a value line of spas (used to be Sweetwater, now just branded as a Sundance 700 or was it 750 series), Hot Springs has their value line, and so do others. You may have to come up just a little bit more, but you'll be getting good value for your money vs buying a spa brand that is less known- more risky.
If you do persue the used spa, and if it's being offered by a private party, try really low balling the price. Unless there is a lot of demand for this type of thing in your area you may have some leverage to save even more. Consider that you still need to have the spa moved- and a pro would charge a few hundred for that alone. Plus the mentioned risk of no warranty, etc...