Hi Marcus,
If you are on SPS 06, potentially you can use something like:
call "mySchema"."MyProcedure"(Param_1 => 'context_p1', Param_2 => '', Param_3 => '', Param_4 => '');
But for the behavior to make it optional, you can also implement that logic in the procedure code by checking the value of the received parameter and then deciding to use it or not.
Regards,
Ravi