Factors to be considered while replacing dead cells with a new 18650 Li-Ion cell in a Laptop
I think that besides the 4 cells, the laptop battery also contains a controller for managing charging, discharging, etc. It also monitors the temperature of the cells and is responsible for safety in general. This is why I advise against replacing individual cells, especially with different ones. You are risking damaging your laptop or even setting it on fire and the money you will safe are not that much.