Let the 1L ferment right out, then chill it for a few days to get all the yeast to clear down. Allow to rise back to pitching temp on the same day as making 4L of 1.040 wort from DME and chill it to pitching temp. Decant all but the last bit of 'beer' from the 1L starter, swish well, and mix with the 4L in whatever you're going to put it in. Ferment right out, chill down, bring back up to temp on brewday; once your boil is on, decant, restart with a bit of cooled wort from early in the boil, finish your brew, pitch. I hop I haven't missed anything too critical there...