FP4 1:2:0:1 (other examples: binary32 1:8:0:23, 8087 ep 1:15:1:63)

S:E:l:M

S = sign bit present (or magnitude-only absolute value)

E = exponent bits (typically biased by 2^(E-1) - 1)

l = explicit leading integer present (almost always 0 because the leading digit is always 1 for normals, 0 for denormals, and not very useful for special values)

M = mantissa (fraction) bits

The limitations of FP4 are that it lacks infinities, [sq]NaNs, and denormals that make it very limited to special purposes only. There's no denying that it might be extremely efficient for very particular problems.

If a more even distribution were needed, a simpler fixed point format like 1:2:1 (sign:integer:fraction bits) is possible.