What is this for? Gaming? And what are your system's complete specs?
You might struggle to find substantially better CPU performance for $300, since any worthwhile upgrade will also require a new motherboard and DDR4 RAM. For most tasks, including most current games, the i7-4790K is still a pretty decent processor, and should be roughly on par with the current locked i5 processors, which offer 6-cores but lack SMT (hyperthreading). AMD's competing mid-range processors like the Ryzen 3600 have 6 cores with SMT, but much like those current i5s, they won't be much faster than what you have on a per-core basis, so you won't likely notice any substantial performance difference in anything that doesn't utilize a lot of cores. Intel will likely be launching new processors within the next few months, but aside from restoring SMT to better compete with Ryzen, I don't think they will be significantly improving performance at most moderately-threaded workloads.
Depending on what hardware your system currently has, and what you use it for, you might be better off putting that money toward other components, like a graphics card. or simply holding onto your money until there are better processors available for the money.