Create a term reference with a select list selecting one at a time
You can do it using Field Collection module

Once enabled, go to your content type and add a new field of Type: Field Collection and set to 4 values.

Then go to /admin/structure/field-collections and click on manage fields and add your select list field here and set value to 1.

Result: 



If you don't want to allow users to set the same value, then you will need to use jQuery to hide value "X" in the 2nd drop down if the 1st drop down is set to value "X"