you can install i7-4790 in 3020 Micro but Dell has designed and validated only low power 35W Intel 4th gen cpu for Micro due to poor chassis air flow/cooling capability. list of Haswell cpu (Intel 4th gen)  Related Haswell CPUs for socket 1150 (cpu-world.com) when you install a 65W cpu there is a chance of overheating because the Micro cooler and chassis air flow are inadequate to handle the heat load.  you may or might hear loud cpu fan all the time and in the worse case PC may shut down or throttle if cpu overheats. 3020 Micro bios does not block the 65W cpu.  It is the practical cooling limitation of micro case that limits use of 65 w cpu such as i7-4790 in Micro. optiplex-3020-micro-technical-spec-sheet.pdf (dell.com) Intel® 4th generation Core™ i5 Quad Core (84W for MT & SFF, 35W for micro), Core™ i3 Dual Core, Pentium® Dual Core and Celeron® Dual Core (65W for MT & SFF, 35W for Micro
is a intel core I7 4790 compatible with OptiPlex 3020m    edit: if not could someone show me CPUs that are compatible with OptiPlex 3020m  because I have a core i7 intel 4790 and it the CPU downclocks it self when running anything I've been through everything else so its maybe because its not compatible.    
OpitPlex 3020M Cpu upgrade