You can get really good deals on intel 10th or 11th gen right now, or ryzen 3000 series.
The i5 11/10400 and ryzen 5 3600 are both very good upgrades for you.
Though sadly you will need new ram and a new motherboard.
Newer chips although faster are not the best bang for the buck so 1 generation behind the newest seems to be very good for price-performance.
and I wouldn't get a lower end chip than those.
even the fastest 4 core processor would not be that big of an upgrade in my opinion and going 6 cores is a much more meaningful upgrade, so i7 4770-i7 7700k are not gonna be that big of an upgrade.
(Unless you can get an i3 12300 a motherboard and ram for cheap*)
i5 8400-9400 would be okay, but I think they wouldn't even be that much cheaper than 11th gen that is quite a bit faster.
*it is a 4 core cpu which isn't good, but it is the fastest 4 core cpu in existance probably, and I think it will be fast enough to be a meaningful upgrade, but it is the only 4 core to be so.