It depends on the particular file and the codecs, resolution and compression that has been used. It can also depend on just what application is used to play the file.
Generally, if you convert a video file in any way, such as changing its resolution or the specific codecs it is using or the file type (mp4 or wmv), then the resulting file will be slightly worse than the original as the conversion process must extrapolate as part of the conversion process. This normally means some loss of detail.
Finally, MP4 is a container specification. It does not define exactly what codecs have been used inside the file, so two different MP4 conversions could produce different results, and in some cases certain player applications may not handle the specific codecs used in an particular MP4 container. This can mean that those applications may play sound but no image, image but no sound, or simply refuse to play at all.
I hope this helps.