Replace 3.7V flat tablet battery with a 18650 cell
Where did you read the battery specification about 3.7V +-5%?

Usually, All these batteries are same with 4.2 Vmax.

You will need more powerfull charging circuit to handle higher current of 4 batteries, and balancer board also.

Maybe you will have to deal with protection and identification circuit, if the original battery has more than two wires (NTC, version).

Edit:

No need balancer for parallel connection.