What you should be doing is this
User selects something from combo 1
Set the rowsource for combos 2,3 & 4 to ""
set the rowsource for combo 2 to cascade from 1
User selects something from combo 2
Set the rowsource for combos 3 & 4 to ""
set the rowsource for combo 3 to cascade from 2
User selects something from combo 3
Set the rowsource for combo 4 to ""
set the rowsource for combo 4 to cascade from 3
What you should not be doing is
Letting a user select from a combo box unless the preceeding combo has a value in it.
Step though each stage to validate your coding.
David