Install the latest VirtualDub (the stable version 1.10.4) but you still can't import MP4 files for lacking required VirtualDub codec pack. If you expect to load MP4 videos, you need to download VirtualDub MP4 plugin or VirtualDub MP4 codec. To fix this problem, just follow my steps:
Update the Plugin Folder to Import MP4 File
1. Download VirtualDub FFmpeg Input Plugin to help you open mp4 in virtualdub
Go to: [https://sourceforge.net/projects/virtualdubffmpeginputplugin/] or visit: [http://codecpack.co/download/FFInputDriver.html] to download VirtualDub MP4 plugin.
2. Replace plugin folder
Unzip the .zip file and drag the plugins64 or plugins32 folder to the VirtualDub installation directory (based on whether you installed the 32-bit or 64-bit version of VirtualDub) to replace the original folder with the same name.
3. Open or edit MP4 in VirtualDub now
Launch VirtualDub and you will find that you can add MP4 videos without hassle.
Attention: For those who want to take VirtualDub save as MP4 files, there's no related direct output setting but you can follow this little trick: Save as AVI > change the file extension from avi to mp4 > then hit "OK". Now VirtualDub exports MP4 file for you.