That's a tough question to answer. Looking at Jamil's "Yeast" book on pg 143, there is a propagation table.
To get 400B cells, you'd need to make a 4L starter using 2 smack packs. The trick is that the correlation between the initial amount of yeast and the the amount of starter you make is not linear.
You'll need to make a small starter of about 250ml of 1.040 wort for your slant. Ferment that on a stir plate for 24 hours, then add that to 500ml of 1.040 wort and ferment another 24 hours. Double like that every 24 hours until you have 4L. You'll have in the area of the 400B (healthy) cells you're looking for.
You need to be extremely careful to keep everything sanitary though. And ferment at about 20 - 22C.
Hope that helps.