It seems that xspf to mp4 conversion would essentially represent export or conversion of .xspf playlists to MP4 multimedia format, something that is not possible. Playlists are just special text files that contain sequence of multimedia files to be played using a media player. The only thing you can do, is to open the .xspf file using a text editor and check the file paths to the audio files, but this might be useless for playlists downloaded from the internet.
Open XSPF file Open MP4 file