Each term indicates differently on MT4 trading platform. Please refer to the following explanations.
Balance: Total amount of funds in the account after the profits and losses from all trades have been realized.
Equity: Balance plus/minus the profits/losses of open positions.
Margin: The margin level needed in order to hold open positions.
Free Margin: Equity minus the Margin held.
While trading on MT4 trading platform, you will see the numbers(amount) of each parameters at the bottom of the platform.
For the calculation of each, please refer to the calculator on the official website of XM.