4µS sono i step del micros()
https://www.arduino.cc/reference/en/language/functions/time/micros/:
On 16 MHz Arduino boards (e.g. Duemilanove and Nano), this function has a resolution of four microseconds (i.e. the value returned is always a multiple of four).
Percui un incertezza di 1 unitá di tempo corrisponde a 4µS.
Ciao Uwe