Mean of standard deviations across columns?

I'm looking for an (array) formula that will take the mean of standard deviations across several columns given the value of another column. For example, I would have

A B C D
2 4 2 1
2 2 2 1
4 2 1 0

I am looking for a formula that will take the mean of the standard deviations of A B and C IN EACH ROW given that column D contains value 1.

(Note that I am not trying to take the standard deviations of all the values in columns A, B and C where the value in column D is 2. I'm trying to take an average of the standard deviations. I have not succeeded in finding an array formula that will do this.

Thanks again for everyone's help on previous problems!