It took me a while to "get" the not gates (the prism ones on the right of figure 4). If that's what you're stuck on, note that the bottom half of the right two

It took me a while to "get" the not gates (the prism ones on the right of figure 4). If that's what you're stuck on, note that the bottom half of the right two diagrams can be seen "from underneath looking into the open trapezoid" (left of the two rightmost) or "from above looking at outsides of (truncated-pyramid-like) trapezoid" (right of two rightmost). Keep staring...