I can see the most important values being:

   ± 0 (infinitesimal)
   ± 10^-2n
   ± 10^-n
   ± 1 (unity)
   ± 10^n
   ± 10^2n
   ± infinity

For fp4, this leaves 2 values. Maybe one of them should be NaN. What should the other one be?