It is only possible to log into one account at a time using the MetaTrader 4 app. However, you are able to switch between your accounts. To switch between accounts:

1. Go to 'Settings' and the top item should show your current account.
2. Touch your current active account and a new screen will appear showing your other available accounts.
3. Select your desired account, or select the ‘+’ symbol at the top right to add a new account.