Is it possible to calculate/estimate the storage space a certain duration of .mp4 video files will take up?
No. 

The MPEG4 video codec allows for a large ranges (easily a dynamic range of 50 for "useful" quality) of compression ratios for the same raw video material. The same applies to audio compressors.

And of course, there's simply video material that is easy to compress, so using the same type of quantization, motion prediction and frame structure, some videos will compress way better than other.

In fact, in many cases, you ask your encoder to aim for a target average bitrate even, so that you don't have to estimate anything, but set the amount of bits per second. If you have a technical necessity to know the bitrate beforehand, that would be the way to go!