Oracle Forms

hi am in form 11gr2 how can I do this validation

I try this

 if :some.field1 is null then
  if :some.field2 is null then
  if :some.field3 != null then
   if :some.field4 != null then
  message('The field is Mandatory');
    raise form_trigger_failure;
  end if;
  end if;
  end if;
  end if;

  field3  ********

  field4    ********

  field1  ******

  field2    ******

  validate when commit

  requrement

  if field3 and field4  are null than

     dont validate field1 and field2

     BUT

     IF THERE IS VALUE IN field3 and field4  THAn VALIDATE

     field1 and field2