Modelling current to remain in particular range for fixed simulation time
Simplest way to implement this is to use $random and compare to four thresholds.  $random will return a random 32-bit integer from 0 to 2^32-1, so just segment this into four chunks, see which one the result falls in to, and set your signal accordingly.  Evaluate this whenever you want the signal level to change - every clock cycle or n clock cycles, or on a delay that's independent of the clock.