From the HOME menu tab use ADD FILE icon on the toolbar to import mp4s to the Videopad media bin. 
There are many ways to isolate a portion of the clip. Perhaps the simplest is to drag the clip to the timeline, if it isn't there already.
Move the red cursor to where content should begin and click the scissors. 
to split the clip. Or click the SPLIT button under the preview window.
Do the same at the end of the desired content. Now right-click the areas to the right and left of those splits and DELETE. What's left is only what you want. Press the left mouse button and drag that clip to the far left of the timeline, if it isn't already there.
On the toolbar click EXPORT | Video file. The default mp4 settings are likely okay. Note where the clip will be saved.
You can do this with each of your old mp4s individually, or make separate sequences for each...
Click the + sign to the right of the sequence 1 tab that's above the thumbnails. 
Each sequence can be exported separately.
No need to close VP. Different clips can be imported into each sequence. Or the same clip can be used, cutting out the unwanted parts.
For more see this.