Decimation X3.5
Decimation X3.5 was skipped for Decimation X4.
Development on Decimation X3.5 started in 2013 and our development evolved enough to name it Decimation X4.
2022-JUL-27 UPDATE:
Matthew found rare footage of Jason playing an early stage prototype of Decimation X3.5 at GDC 2014 San Francisco and posted it for historical sake (see below). Decimation X4 remains on hold.
Rare gameplay footage of Decimation X3.5 prototype at GDC San Francisco 2014.