[parsimo2010]

In standard FP32, the infs are represented as a sign bit, all exponent bits=1, and all mantissa bits=0. The NaNs are represented as a sign bit, all exponent bits=1, and the mantissa is non-zero. If you used that interpretation with FP4, you'd get the table below, which restricts the representable range to +/- 3, and it feels less useful to me. If you're using FP4 you probably are space optimized and don't want to waste a quarter of your possible combinations on things that aren't actually numbers, and you'd likely focus your efforts on writing code that didn't need to represent inf and NaN.

  Bits s exp m  Value
  -------------------
  0000 0  00 0     +0
  0001 0  00 1   +0.5
  0010 0  01 0     +1
  0011 0  01 1   +1.5
  0100 0  10 0     +2
  0101 0  10 1     +3
  0110 0  11 0     +inf
  0111 0  11 1     NaN
  1000 1  00 0     -0
  1001 1  00 1   -0.5
  1010 1  01 0     -1
  1011 1  01 1   -1.5
  1100 1  10 0     -2
  1101 1  10 1     -3
  1110 1  11 0     -inf
  1111 1  11 1     NaN