4mph? Have you measured that for you and your friends?
I've just checked a few websites (including health ones and wikipedia) and they all agree that a brisk walking pace is 3 to 3.5mph e.g. Wikipedia "An average walking speed is about 5 km/h (3 mph), although this depends heavily on factors such as height, weight, age and terrain." Also http://www.bellaonline.com/articles/art20257.asp
Average walking speed of 4mph? Really?
I've just checked a few websites (including health ones and wikipedia) and they all agree that a brisk walking pace is 3 to 3.5mph
e.g. Wikipedia
"An average walking speed is about 5 km/h (3 mph), although this depends heavily on factors such as height, weight, age and terrain."
Also
http://www.bellaonline.com/articles/art20257.asp