From the Houston Chronincle:

The Astros cleared space on their 40-man roster Wednesday by outrighting four players to Class AAA Round Rock: righthanders Dave Borkowski, Jack Cassel and Ryan Houston and infielder Jose Castillo.

Borkowski and Castillo have the right to refuse the outright assignment and become free agents, while Cassel and Houston automatically become minor league free agents based on their minor league service time.

The Astros’ 40-man roster stands at 36.