Using two classes to calculate splitting criterion in decision tree

Hi

I have 4 classes on decision tree problem (A,B,C,D). However, I would like to calculate the probabilities used in the splitting criterion of fitctree with just two classes at the time (A, C).

For instance

P(A) = A /(A+B) P(C) = C/(C+D)

and use any of the splitting criterions calculated only on A and C. The role of class B and D is to modify the probabilities of A and C at each step.

Any help would be appreciated it.

Thanks,

G