What happens when you play a MP4 file? Is there an error message that you see?
In the meanwhile, please follow these steps to resolve your issue.
1. Open RealPlayer.
2. Click the RealPlayer logo, then Preferences.
3. In the Category pane, select Hardware.
- Under Video Card Compatibility, move the slider bar Most reliable.
- Click OK twice to save the changes.
- Close out of RealPlayer, restart it, and sign back in.
- If this does not work, set Video Card Compatibility back to its default.