The new Ryzen 3000 series is hard to beat right now for the price to performance. And motherboards, atleast the B450 are pretty cheap(although BIOS compatibility might be an issue) although a new X570 can be had for the price you were willing to pay for that old mobo.
The 4790 isn't bad as long as you can find a Mobo for cheap, which again used shouldn't be an issue. Check Ebay and Facebook Market. I wouldnt spend more then 40 or 50 on that old of a mobo.