Multiple minecraft accounts and cost?

First, what you'll need to do is actually buy the game. On android, it is $6.99 (USD), and on PC, it is $26.99 (USD). You need to buy minecraft for 4 devices, to play with 4 people. After that, you can sign in with a microsoft account by selecting "Microsoft Account" to the bottom right of the home screen. Then, follow the instructions from there. You need to have 1 Microsoft account for each device, and if you dont have them already, you can find out how to make them by following the instructions (you will be able to find out how to make an account on the Microsoft Account screen).

Microsoft accounts are free to use.