Bitrim will be appearing on Steam Greenlight!
Steam Greenlight will be available around the end of August, and Bitrim should have a page there within the first few days. Alpha 4 will definitely be available on that date, or before.
Remember: You can pre-order Bitrim by buying the Prologue version above. You'll get an offline client version (instantly) and full access to Bitrim when it's released, including alpha/beta access.