I used a PC with only 4gb of RAM up until very recently- and it was ok. The one single upgrade for a laptop that is VERY worth it is an SSD- physical hard drives are slow, unreliable, and get worse with age. An SSD would make the machine faster than new!
However.... 8 years is OLD for a laptop. If it were in perfect repair but old, it could make sense to upgrade- or you could buy a much nicer and newer starting point with a good screen, good touchpad, etc. The price difference would likely be minimal, or even save you money. I would personally recommend a 2015 macbook pro. The 2015 was the last year before they brought out the crazy thin keyboard that broke all the time, removed a bunch of ports, etc. Mac people I know LOVE the 2015 macbook line. I would then look for one with a decent battery according to the seller, at a good price point. It would probably be cheapest to get a 4gb RAM model with a hard drive- then, upgrade the ram if you want, certainly put in an SSD, and... enjoy the laptop. The 2015 models had non retina and retina versions- a low and high res screen. However, I think it MAY be the case that the retina models are thinner and with better battery life due to a chipset upgrade- but that is from shaky memory. You are somewhat OK with your old 2011, so getting a good condition pre-retina 2015 might just be fine for you. (They are cheaper, and I am mostly suggesting upgrading because you would have to buy too many parts to properly restore your old one, not that it cannot do what you need. That, and a newer model will give you more years of OSX upgrades)
I personally use linux/windows, and don't own a mac, so do your research, but I seriously considered buying a 2015 macbook and upgrading it before I found a great deal on my current thinkpad. Price out buying a ready to go 2015 model in comparison to buying a low end one and upgrading it- it may be worth it.