If you wish to buy new I think the Epson V700 or V750 is probably the best for your price range. If you are happy with your current scans then you should be happy with the bigger Epsons for 4x5. I have an older 4870 and it's decent for negative scans enlarged 3 to 5X. That should give you very nice 16x20 prints. I would start out with the Epson V750 and try the fluid mount holder. Film flatness and focus point are the weak areas of the Epsons, and those can be fixed.
There aren't a lot of other options for 4x5 besides the Epsons that I know of. A used drum scanner will give you better quality, and with patience can be found for your budget as well. But drum scanners require more learning, support is more difficult to obtain, and they are considerably larger. Most are also 15 or more years old at this point. If you like to play with old computers and equipment getting a drum scanner can be fun, but it probably isn't a good option for most people.