Some useful information you'll find here:
http://office.microsoft.com/en-us/access-help/set-a-default-value-for-a-field-or-control-mdb-HP005188852.aspx[
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You don't need to set default value for field in database table, because, you can always set "default value" in query:
PARAMETERS p1 INT, p2 INT, p3 INT, p4 INT;
INSERT INTO YourTable (col1, col2, col3, col4, col5)
VALUES(p1, p2, p3, p4, (p1+p2+p3+p4)/4);
But i'm wondering why do you want to store average value, if you always can calculate it (based on col1-col4 values)?
SELECT col1, col2, col3, col4, (col1+col2+col3+col4)/4 AS col5
FROM YourTable