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, go to 'Settings' and the top item should show your current account.
Touch your current active account and a new screen will appear showing your other available accounts.
Select your desired account, or select the ‘+’ symbol at the top right to add a new account.
Where can I download the MetaTrader 4 iPhone app?
How do I log in to my account using the MT4 iPhone app?
How do I place a trade on the MT4 iPhone app?