I'm gonna go ahead and say go all out on your short commute. Ride it like your doing drag races between every light! If you like going fast, make the most out of the 4 miles. When I was doing 3 miles, I was going flat out and it was fun, and I also had days that I dogged it (especially windy days). My fastest commuting times were under 12 minutes, but it could take up to twenty with lights and wind (which are the biggest factors on my now longer commute as well).