Some of it depends on the age of the yeast, i.e. production date, volume of wort, and the OG of the wort you are pitching into. If the yeast is old, you can do a very low gravity starter, approx 1.020, 1-2 liters and let it ferment out. Then chill and decant and then step it up to 4L. Increase the gravity to 1.030-1.040. Then chill and decant again. Then what I like to do is take some of my wort that will become beer and add 1-2 liters to the decanted yeast. Get it close to the pitching/ferment temp for a couple of hours to get happy and then pitch it. Even if you underpitch it should take off pretty quickly since you are pitching active yeast. Also, you could always buy another vial of yeast if you are short on time.