Maximum CPU for the 9020:

MT: i7-4790 (it can also take a i7-4790K but only if you replace the power supply with an aftermarket one and a 24 to 8 pin adapter, however, the mobo doesn't support overclocking)

SFF: i7-4790

USFF: i7-4790S

Micro: i7-4785T

You will see a slight performance increase going from a i5-4590 to an i7-4790.

according to intel's own site it is LGA 1150. But there's probably still BIOS/chipset issues.

Intel Core i75775C Processor 6M Cache up to 3.70 GHz Product Specifications

Is it even worth it, the performance difference between 4th and 5th gen? Would it be better to go to i7-4790 instead? I'm seeing those for about $60. I have i5-4590. I don't know how much of a difference I'd be getting. Already 4 cores, just higher frequency and hyperthreading.

i7-4790k is a better cpu than i7-4790 in that the former base frequency is 4.0ghz.  4790 is 3.6 GHz.  Consider the k a premium binned silicon favorite chip already factory overclocked for you.

single thread performance of 4790k is better than 4790 (2500 vs 2200).  cpu mark is 7200 vs 8000. https://www.cpubenchmark.net/compare/Intel-Core-i7-4790-vs-Intel-Core-i7-4790K/2226vs2275

downside of buying used -k cpu is unknown history.  If seller said it has never been overclocked, walk away because most likely that is untrue (unless seller is a trustworthy buddy and original owner).  If -k cpu has always been used with a liquid cooler and seller shows evidence of taking good care of it, maybe worth considering. Not worth it to buy new 4790k in 2022.

I do not think you would need to upgrade psu or need adapter for 4790k.  The psu requirement of 4790k is the same as 4790 which is already supplied by 9020 MT or SFF.  Correct that neither chipset nor bios supports -k cpu overclock.  Pancake cpu cooler and lack of VRM cooling do not support overclock either.

Ha-ha there's a difference between cipset and socket