Complex sentence: The Falcon 9 Block 4 is a version of the Falcon 9 rocket expected to be launched in 2017, as a transition between the Falcon 9 Full Thrust (Block 3) and the Falcon 9 Block 5.
Split into simpler sentences: The Falcon 9 Block 4 is a transitional version of the Falcon 9 rocket expected to be launched in 2017. It is expected to be a transitional model between the Falcon 9 Full Thrust (Block 3) and the Falcon 9 Block 5.