To calculate Omar's average speed, you need to divide the total distance he covered by the total time it took.
Total distance = Distance from marker 1 to marker 4 = 4 miles - 1 mile = 3 miles
Total time = Time taken from marker 1 to marker 4 = 6 minutes + 18 minutes = 24 minutes
Now, convert the time to hours (since the speed will be in miles per hour):
Total time in hours = 24 minutes / 60 minutes per hour = 0.4 hours
Average speed = Total distance / Total time in hours
Average speed = 3 miles / 0.4 hours = 7.5 miles per hour
So, Omar's average speed from mile marker 1 to mile marker 4 was 7.5 miles per hour.