MetaTrader 4 allows you to close only a portion of your open position rather than the entire trade. Here's how to perform a partial close:
Locate your open trade in the Terminal window (at the bottom of your MT4 platform)
Right-click on the trade you want to partially close
Select "Modify or Delete Order" from the context menu
In the Order window that appears, select "Market Execution" from the Type section
In the Volume field, enter the amount you wish to close (not the amount you wish to keep)
Click the yellow "Close" button to execute the partial close
Example: If you have a 1.0 lot position of EUR/AUD and want to close 0.03 lots, enter 0.03 in the Volume field. After execution, your remaining position will be 0.97 lots, which you can verify in the Trade tab of the Terminal window.
Close By Trades in MetaTrader 4
The "Close By" feature allows you to close opposite positions of the same symbol against each other:
Open the Terminal window (Ctrl+T)
Go to the Trade tab
Right-click on one of your opposite positions (e.g., a buy position)
Select "Close By" from the menu
In the dialog box that appears, select the opposite position (e.g., a sell position)
Click "Close" to offset the positions against each other
If the positions have different volumes, the smaller position will be closed completely, and the larger one will be partially closed, with the remaining volume still open.