I have a video file on my Drive which is very old but I am not sure whether it's infected or not. Is it safe to view this MP4 on Google Drive (no downloading only viewing it in the Drive app)?
The file will be downloaded to your machine's temporary files and viewed with a viewer in your browser (the Drive app). If there is a vulnerability in your browser, there is a chance that your machine will be infected.
If you are using an up-to-date browser, and since the file would have an old virus, if any, you have a low chance of infection.