\[b = |a| - 4\]
\[a = - 5:\]
\[b = | - 5| - 4 = 5 - 4 = 1.\]
\[a = 0:\]
\[b = |0| - 4 = 0 - 4 = - 4.\]
\[a = 4:\]
\[b = |4| - 4 = 4 - 4 = 0.\]