It appears that both .pls and .b4s files are multimedia playlists, typically with a list of songs played randomly or in a sequence in supported media player.
Many media players offer the function to simply re-save playlists to a different format, so that's one of the easiest ways how to convert pls to b4s. Another option would be to use dedicated playlist conversion programs like Playlist Manager which can help you with pls to b4s conversion.