[MT4] Yes, if you want to close part of the trading volume of your open position, you can specify the trading volume you want to close and close only that amount. Double-click on the relevant order, select "Market order" for the order type on the order screen, enter the desired transaction volume for the quantity, and click "Settle" (yellow button).
[MT5] Partial payment is not available. If you wish to make a partially close a position, please use the desktop version of MT5 or the MT5 mobile app.