I'm in a similar position in my new band which is a four piece (Drums, singer, guitar and bass), and I had this thought too. I decided in the end that the 'drop' you speak of actually goes a long way to emphasising the solo, and that I liked the way it change the dynamic of the song.
If you were set on filling it out though, I would say your best bet would be some form of tubey grit, to beef up your tone and spread out into the sonic emptyness.