I say it really depends on your financial situation. Upgrading to 4K is a noticeable Improvement. But, it's not life-changing. I think it should only be done if you will not miss the money.
If you do it, I don't think you want to go under $500 unless you're talking about something like a 42 inch TV. Maybe you could get a halfway decent 50 inch unit at that number if you catch a good sale. If you want to get better than that, I'd wait until you feel comfortable spending a little more money so that you can get something good.