Significance of 4 stimulus cards in the Wisconsin Card Sorting Test?
I cannot tell you why they chose four cards, but I do know that it is perfectly fine to use three, (e.g. Niv and colleagues, 2015).

To give you more info on why the WCST is used in the study by Niv. The study wanted to see how you could best model learning (and relearning) multidimensional information, to be able to better understand how people make sense of the multidimensional information provided by the real world. Therefore, they had participants perform the WCST with three cards. Multiple reinforcement learning algorithms were then fitted to the behavioral data, and they found that a reinforcement learning model that includes decay (as to model memory decay) was best able to do so. 

I believe it would have been perfectly fine to have used four cards in that study as well. It would make it more difficult to learn on which dimension to focus, which would probably also be modeled. 

There are also variations of the WCST (e.g. Catherwood and colleagues, 2014). In that study they instead of took apart four dimensions and showed a 2x2 matrix with each unit depicting a different dimension. 

In short

Depending on the goal of your experiment, you can decide how you would like to use the WCST specifically.