Download your EA, and paste it into the ‘Data Folder/Experts/Advisors’. You can access it by going to ‘File’ in the upper left corner and then clicking on the ‘Open Data Folder’. Restart your MetaTrader 4 for your changes to take effect.
You can find custom EAs on the MQL5 website. Click https://www.mql5.com/en/code/mt4