how much it costs to install an additional 4GB memory?

Are you talking about adding memory after purchase of the computer? If so, this has not been possible since about 2012 before the introduction of the retinal display MacBook Pros.

If you have an older, pre-retina display MacBook Pro you can get memory from OWC.

If you have the default hard drive in that machine - a 5400 rpm drive - then more RAM will not help that much. But replacing that is easy in a mid 2012 non retina, I did it putting in an SSD and a total of 12 GB RAM (replace one stick with an 8 GB) and it was responsive and quick. They have great install videos on their site (OWC).

Hi,

If you want to speed up your MacBook Pro, upgrade RAM, you can upgrade till 16GB(two 8GB sticks) and replace HDD with SSD, ordinal 2.5inch SATA is compatible.

https://eshop.macsales.com/shop/memory/Apple_MacBook_MacBook_Pro/Upgrade/DDR3_1600MHz_SDRAM

https://eshop.macsales.com/shop/ssd/owc/macbook-pro/2012

Should speed up your system. Think of "storage" as file cabinets and "memory" as desk space ... you can alway do things faster when the work is on the desk already. Also ... do not worry if you buy 8 GB RAM and the system uses 7 GB of it ... Apple designs MacOS under the rule "unused RAM is wasted RAM".

If it is not a retina display model check the OWC web site for available memory upgrades.

But fwiw, I would not spend any money on a vintage/obsolete old computer.