All good advice above.
4 km/2.5 miles is very short. The car definitely doesn't get warmed up in that distance so you won't get anything like EPA from it. Hypermiling techniques will help but there's only so much you can do with a cold engine. But shucks, with those kinds of distances you're hardly burning very much gas anyway. Round trip = 5 miles = .25 gallon per day at 20 mpg.
I put a little pad type oil pan heater on my '97 Civic (same generation car as yours). On the vertical end part of the oil pan, driver's side.
see the heater pad It helps, but the kind that goes into an engine block freeze plug probably does a more thorough job. This one does help though, and is pretty easy to install. Uses only 125 watts. With a short ride like yours I'd use the heater year round.