In the NES version of Life Force, since the original stage 4 was instead stage 2, a completely different level was made for stage 4. It is known as the Organic Speed Zone. It takes place vertically, and the walls and background are, of course, organic.
The stage starts normally, but eventually, when the walls become brain-like in appearance, the scrolling will speed up by a lot, and the stage becomes all about dodging wall in time. Eventually, the scrolling will return to normal speed. The end of the stage features what appears to be a giant human ribcage, which the tips will shoot lasers. After passing through it, the boss is a giant human skull, known as Giga.