I wrote half a song a couple years ago and never finished it, but it used a similar progressions: Em, C, B, Bb.
Get to know all of the notes in each chord and try some variations on the chords to see what fits, might help with a melody. For example I could hold a high Bb on the last chord and get a nice diminished harmony when Em comes around again.
But I'm no musician, so you can ignore me if it pleases you to. =)