The commands SET PROCESS VARIABLES and GET PROCESS VARIABLE allow you to respectively set or retrieve the contents of a variable in a process from a different process. For example, if you want to retrieve from 4D Client a process variable in a stored procedure, you need to pass the process ID of the stored procedure in negative mode. If that variable was an interprocess variable, you could just provide -1 as the process number since interprocess variables are shared by all processes on the server.
However, keep in mind that this requires that both variables do exist in their specific processes. For example, you cannot set a variable that does not already exist in a process.