I would take a look at Vizio E-Series (This model is the E43-F1), decent quality 43” TV/4K as you requested, stellar reviews for the majority and its priced at $309.00 ‘On sale’ under your budget of $500.( Some the key points is it includes Better motion handling, Higher contrast ratio and the U.I. is said to be easy to navigate).
Personal experience, Vizio had quality control issues years back, but they really have capitalized with improved chipsets and higher quality display panels, and I think they are one of the better valued TV manufacturers for the money spent.
According to the link, Best Buy currently has this in stock.