Can a complex primary key made up of 4 fields have only one of this fields set to AUTO INCREMENT?
You can do this:

CREATE TABLE mytable
(
    `ID` MEDIUMINT NOT NULL AUTO_INCREMENT,
    `C1` MEDIUMINT NOT NULL,
    `C2` MEDIUMINT NOT NULL,
    `Year` MEDIUMINT NOT NULL,
    PRIMARY KEY (ID),
    FOREIGN KEY (C1, C2)
            REFERENCES othertable(pkcol1, pkcol2)
);

ALTER TABLE mytable ADD UNIQUE INDEX(`C1`, `C2`, `Year`, `ID`);


Adding the ID column to the unique index will guarantee that you can have the same values for C1, C2 and Year in your table. If you don't want that, remove ID from the ALTER TABLE statement.

The syntax of the FOREIGN KEY is for a multiple column key in another table called othertable. pkcol1 and pkcol2 would be the corresponding columns in that table.

ID would be your fourth column in the question example code you presented.