row and column formatting

I am trying to do the following:

column a – about 1/3 across
column b – about 1/3 across
column c – last 1/3. Column c is very long.
column d – about 2/3 across under column a

So basically the long column c goes a long way down the page, with multiple rows of columns on the left side.

Here is column a here is column b here is column c
still a still b still c
New column d goes across under a and b still c

Hope the last section worked.

Thanks,
Doc