Without any code or some more clues, any advice would be hypothetical (you have more clues at your disposal so will know better what the issues are). I would advise against your JRE6 idea, RaspberryPI4 might at a pinch work with jdk8 but all recent distros support jdk11+. Indeed I have had good success with ManjaroARM(64 bit) on the RaspberryPI4.