PHEV solution for my new commute?

I will soon be going from a 40 mile one-way commute (OC to Burbank) to a 4 mile commute (surface streets) in a month, with a new job. I'm so excited about the shorter commute, the extra time for sleep and family, woot!

With my long commute, and defensive driving habits, I have gotten 57 to 58 mpg out of my 2006 Prius. It also helps that everything slows to a crawl during rush hour traffic. My new commute is a straight shot down one main thoroughfare, which has a 40mph speed limit.

What can I do to keep my MPG's up? I was contemplating using just an EV switch, but is there a way to top off the battery each night via plug-in? I think a true PHEV system, while great for the environment and mpg's, will cost so much as to make the ROI negative in terms of fuel savings. Or is the best value just to suck it up and take a 10+mpg hit, knowing I'm still doing better than any other car I might drive.