You are required to create a Python class named BankAccount that models a simple bank account in a file called test$\_$bank$\_$account.py $.$ The class should be fully encapsulated:

a$.$ Class: BankAccount

b$.$ The BankAccount class should have the following private attributes and public methods:

i$.$ account$\_$number : A string representing the unique account number.

ii$.$ account$\_$holder: A string representing the name of the account holder.

iii.$$ balance : A float representing the account balance.

iv$.$ account$\_$type: A string representing the type of account $($e$.$g$.,$Savings$,$Checking$).$

v$.$init $($self$,$ account$\_$number: str$,$ account$\_$holder: str$,$ balance: float, account$\_$type: str$)$: Initializes a new BankAccount object with the provided account number, account holder name, initial balance, and account type.