AviDemux has some easy scrolling tools. Bring in your mp4 file, set the A point at the start of the timeline, and since you want to split this mp4, manually drag the cursor on the timeline somewhere close to the middle.
Then use the control that I've pointed out in this picture. It will advance the cursor to the nearest I frame. Set your B point. This will cut off the back half of the movie (it will not effect your original file, so don't worry about that).
Make a note of where this B point is in time! In this example it is at 01:06:19.809.
Now save this first section of your mp4 file as I explained previously.
To get the second section, bring the original mp4 file back into AviDemux, set your A point at the I frame where you put the B point in the first cut. Now, put the B point at the end of the movie. This will cut off the first half, and allow you to save the second half of the movie.