You can use a White Labs vial a lot more than 4 times too. If you do a 3 stage starter with a stir plate, you only need several drops, but it does take a while and you need a stir plate to get a good cell count.
Day 1, add some drops of liquid yeast into 10 ml starter wort(small container.. I use a little glass cup), 3/4" or 1" stir bar
Day 2, add to 500 ml starter wort, step up to larger container, switch to 1.75" or 2" stir bar
Day 3, add 500 ml starter wort
Day 4, crash cool
Day 5, decant and pitch
I have done this a handful of times with great success. I end up with what would have been around 2 full vials worth of yeast. I'm not 100% of the quantity, but it's enough that you'll need a blowoff tube in about 24 hours or else you'll blow out your airlock or lid.
I am not quite sure how long this can be repeated without eventually contaminating the master vial. I try to open it, pour some and close it as quickly as humanly possible.