Many users are looking for a way to create a video sequence from a set of photos, which is what jpg to mp4 conversion usually refers to. For this you will need a special presentation utility like the Animoto or Time Lapse Assembler for Mac, which have been designed exactly for this purpose.
Another take on how to convert jpg to mp4, might be related to showing pictures as a background during audio playback. MPEG-4 contain support this just fine, so you just have to use one of the dedicated multimedia editors to insert the pictures and embed them to the MP4 file.
Open JPG file Open MP4 fileA multifunctional media converter for Windows
A picture to video converter
A dynamic slideshow maker