You won't see any difference in quality. What you will see is better performance from your computer.
MP4s (h.264) are "long-GOP" files and require more computation to play back and edit; the GOP (Group Of Pictures) consists of a full video frame followed by several delta frames, which represent the differences between that frame and the initial frame, making editing more complicated for the processor. ProRes files are "I-frame," meaning every frame is individually compressed; this makes playback and editing less complicated for the computer.
The other thing you might gain is less artifacting as you apply heavier effects, such as color grading. ProRes is a 10-bit codec, so it won't fall apart quite so soon. Now, your 8-bit MP4s are still limited in the amount of color information that they hold, so expect some blocking on heavy color adjustments and such.
But the primary benefit of transcoding to ProRes (or DNx) is I-frame performance. This is especially important if you're working in UHD or 4k, since those files are "heavier" right out of the gate. In that case, use Premiere's proxy workflow to transcode to ProRes Proxy or ProRes LT for editing. All your exports will still be from the original material (unless it's offline).