VPC plots stratified by more than two variables

[JoannaPeng]

How to create VPC plots stratified by more than two variables?

Specifically, the VPC was created using PsN with -stratify_on=VAR1,VAR2,VAR3,VAR4.

In R, vpc_data=vpc_data(..., stratify=VAR1~VAR3), but this line specifies only two variables. How to modify this line to specify all four variables? (And the combination of VAR3 and VAR4 will be used as a column variable, and the combination of VAR1 and VAR2 will be used as a row variable.)

I tried stratify=c('VAR1','VAR2','VAR3','VAR4'), but got an error message.