Main wallet unspent transaction
All wallets will calculate and show the same amount as the &quot;balance&quot; for an address. In your example, all wallets would show the balance as 4 BTC. There are exceptions if a wallet is faulty or is not synchronised (not up-to-date, doesn't yet know about latest transactions).
If you import an address into a wallet, it will show the correct balance but it will show that amount as &quot;watch-only&quot; or &quot;non-spendable&quot;.
There is no way to discover private-keys from addresses.