It sounds like you're dealing with a pretty frustrating issue. CPU usage while playing Minecraft can vary greatly based on many different factors, like the game's render distance, whether Vsync or an FPS limit is enabled, and which hacks you're using in Wurst.
As a rule of thumb, any hacks that need to process entire chunks, like Search, PortalESP and MobSpawnESP for example, will demand a lot more CPU power than hacks that don't process chunks. This is because they are processing millions of blocks across several CPU cores.
4 GB of RAM seems like a reasonable amount to allocate per Minecraft instance - and making sure the game has enough RAM is certainly a good troubleshooting step, but as you've seen, having more RAM usually doesn't mean less CPU usage, since these are separate aspects of your system's performance.
I recommend you try limiting each instance's FPS or reducing the render distance to see if that makes a difference. Also, review the hacks you have enabled, and consider disabling any chunk-processing hacks unless you really need them.
Hope this helps, and feel free to reach out if you have any more questions!