I have an Imac late 2012 (32GB Ram) on Catalina and FCPX 10.4.8
1-Any chance to get 4K on an external monitor for FCPX timeline or do I have to upgrade the Imac?
2-What about playing AppleProRes exported files (from SSD) on an external 4K monitor?
Using dual monitors with FCPX is easy but your iMac is kind of old and may not support 4K. There are Thunderbolt products from AJA and Blackmagic Design that will output to 4K monitors but they may require Thunderbolt 3. Your iMac might have Thunderbolt 1 ports. You will have to do a little research to find out what are your options.