another starter question

This sunday im going to brew up 20 gallons of spiced christmas ale that i hope to have ready by the holidays. The target OG is 1.070 so its a big beer i have a 5000 ml flask and a stirplate but i will not be able to get to the LHBS until saturday. Mr.Malty calls for 4.56 Liters of starter with 4 packs of yeast. should i pitch the packets in 4 liters of 1.040 wort leave it on the starter overnight and pitch the starter on sunday or let it sit on the stirplate overnight then during my brew session put it in the fridge to cold crash and decant off the wort. Also with such a large starter am i able to get away with not stepping up and just pitching the whole thing at once ? would it help to get my cell count up faster if i add a bit of yeast nutrient to the starter wort ?
Lots of questions i know, this is my first time doing a starter and from what i have read in the search feature i may want to give it more time to do its thing next time just wondering what my best course of action is.

If you plan on the starter going for only a short time (in this case, ~12-24 hours), pitch the entire contents of the starter. You should decant only if you have given the yeast the chance to ferment the starter completely (for the same reasons you shouldn't just cold crash when you hit your desired FG - let the yeast finish their job). Extreme cold crashing could stress yeast further and cause them to give more off-flavors. Gentle cold crashing is ok, although after cold crashing, you would also want the starter to warm slightly closer to pitching temps to avoid shocking the yeast. By all means, add yeast nutrient, just remember to scale down for 4.56 L. As far as stepping up, you bypass that by using 4 smack packs in this case. You couldn't pitch 1 or 2 packs into that same volume and expect the same results.

So, in short:

Smack 4 packs, dump into 4.56 L of 1.040 wort, aerate on stirplate and pitch 12-24 hrs later when done brewing.