Backtesting  in MT4 is a very important advantage you can use to have a basic idea about the validity and accuracy of a trading system , Backtesting is not a final judgment to be sure that this trading strategy or system is what you are looking for but as i said it gives you initial sense and evaluation .. 

The next 2 videos are going to explain MT4 Backtest and illustrating every item in the process of doing your Backtest ..