I take it that I need to manually upgrade it? and once I upgrade to 4.x it will maintain itself after that?
What is the exact procedure to do this and what real benefit should I expect.
Thanks
Here are the instructions that I used to upgrade from 3.x to 4.x. using uboot.
I advise you to not try and keep your configuration settings. Instead just note them down and re-configure the router once it’s upgraded.
The benefits are a whole host of new possibilities, the drawbacks are a whole host of new possibilities 
Benefit you will be running openwrt 21.02 snapshot instead of Chaos Calmer 15.05