The font size is a field level property for comboboxes (which means that it is the same for all fields with that name). You will therefore have to rename the box on page 4 before you change the font size. Then you will have to add some JavaScript (best in the Calculate event) which pulls the value from the main field.
Let's assume that the "main" field is called "myComboBox". In this case, you would add the following code to the "dependent" field:
event.value = this.getField("myComboBox").value ;
It would also be a good idea to make the dependent field read-only, because with this scenario, we don't have the "back connection" which would change the main field value when we change the dependent field.
Now, if you can not make the dependent field for any reason, there would be the workaround to set the font size to automatic, and change the field size(s) accordingly.
Hope this can help.
Max Wyss.