So if the "empty" value in column coming from table2 can just be NULL, you can do a LEFT JOIN
select
t1.col1, --this is A, B, C
t1.col2,
t1.col3,
t2.col4 -- this is the fourth column
from Table1 t1
left join Table2 t2 on t1.Col1 = t2.Col1
see SqlFiddle
If you wanna be sure to join only if the 3 columns are the same, just add conditions to the left join
on t1.Col1 = t2.Col1 and t1.Col2 = t2.Col2 and t1.Col3 = t2.Col3