2 vials into a 4L starter sounds about right. I've always used lager pitch rates for 100% Brett beers and that has worked well. A couple things to keep in mind... Everything seems to take about 3 times as long, so wait another day or so before freaking out. Yes, you will see krausen when using 100% Brett, how much depends on the strain. I've even had some strains form a pellicle in the starter while using a stir plate.
It takes a lot longer this way, but what works for me when using 100% Brett is a multi-step starter. Pitch 1 vial into 500 mL, step up to 2L, then step up to 4-5L. With cold crashing and decanting, this usually end up taking 2-3 weeks. Then when I pitch into my 5 gallon batch, it will finish in a month or two (+/- a few weeks depending on the strain).
Brett Claussenii is one of the slower working strains in my experience, so keep that in mind too, but your patience will be rewarded.