Using Switch Case or IF...ELSE Statement in a Dynamic Function Query

Question

Suppose I have a PostgreSQL function that take inputs and inserts them dynamically into a query:

CREATE OR REPLACE FUNCTION cnms_fy22q2.dynamic_function(
    fyq text)
    RETURNS void
    LANGUAGE 'plpgsql'
    COST 100
    VOLATILE 
AS $BODY$
BEGIN
    EXECUTE 'SELECT
       region,
       state,
       || fyq || as fisc_quarter,
       CASE
            WHEN RIGHT(fisc_quarter, 1) = '1'
                THEN 'A'
            WHEN RIGHT(fisc_quarter, 1) = '2'
                THEN 'B'
            WHEN RIGHT(fisc_quarter, 1) = '3'
                THEN 'C'
            WHEN RIGHT(fisc_quarter, 1) = '4'
                THEN 'D'
            END
       AS code
    FROM schema_' || fyq || '.my_table';
END
$BODY$;

The above does not work because fisc_quarter is not recognized in the query at this point. I am not sure if I could use the input value fyq perhaps in an IF ELSE statement to replace the Switch Case part. Perhaps first adding:

....
DECLARE
    var1 varchar := ''; 

And then

AS $BODY$
BEGIN 
   IF RIGHT(fisc_quarter, 1) = '1'
       var1 = 'A'
   ELSE IF RIGHT(fisc_quarter, 1) = '2'
       var1 = 'B'
   ELSE IF RIGHT(fisc_quarter, 1) = '3'
       var1 = 'C'
   ELSE IF RIGHT(fisc_quarter, 1) = '4'
       var1 = 'D'

What would be the most appropriate way to set the code column based on the input parameter here?