WHERE
t1.field1 = t2.field1 and
t1.field2 = t2.field2 and
t1.field3 = t2.field3 and
t1.field4 = t2.field4

WHERE
 ((t1.field1 = t2.field1) or (t1.field1 is null and t2.field1 is null)) and
 ((t1.field2 = t2.field2) or (t1.field2 is null and t2.field2 is null)) and 
 ((t1.field3 = t2.field3) or (t1.field3 is null and t2.field3 is null)) and 
 ((t1.field4 = t2.field4) or (t1.field4 is null and t2.field4 is null))