Four years isn't particularly old for any laptop, Mac or otherwise.

I won't go through your terminology, just make assumptions what you mean.

Your battery is dead, if you keep the machine, you need to buy a new one. No, it won't improve performance just run the machine longer.

Your fan is running almost continuously, that implies the machine is running hot, most likely due to accumulated dirt and dust in the machine. The machine will try to protect itself by slowing down or even shutting down. Get it cleaned. You can try blowing canned air through the vents you can reach but it may need the cover taking off to do it properly and on Macs, that likely isn't a do it yourself job.

You are going to be struggling if you have only 30GB disk space left out 500 GB. Probably there is a lot of accumulated junk files that have built up on the machine and as you say, the VM may not be helping. First thing to do is go through the disk and clear the junk out. If you can't reach at least 10% free space and preferably 20% then you need a bigger disk. At today's prices, you can consider a solid state disk, which is probably the best performance improvement you can get after cleaning the machine. You may need to get Apple or an Apple service agent to replace the disk.

Resetting the machine to factory state and then re-installing your applications should get you back towards how it was when new, after you've done the physical cleaning and sorted out your disk.

It's still a decent spec machine, I wouldn't throw it away just yet. But cost it out, especially if you need to have a service agent do some of the work and compare against your new machine price. Your choice.