Driving fuel injectors?

Depending on what kind of time frame you are talking it could be done by running a timer.

#1 on at x time
#1 off at x time
#2 on at x time
#3 off at x time
#4 on at x time
#4 off at x time

Each loop can adjust the values that the timer falls on for the on and off times to adjust your output periods. Interupts could be used depending on how time sensitive you need it.