Multiple IF statements within one cell

All,

I am using Excel 2007 and am attempting to write multiple IF statements within a cell. This multi-cell validation would be used to validate that A=B and C=D but A&B do not equal C&D as illustrated below:

A B C D
1 1 1 1 FALSE
1 1 1 0 FALSE
1 1 0 1 FALSE
1 0 1 1 FALSE
0 1 1 1 FALSE
1 1 0 0 TRUE
0 1 1 0 FALSE
0 0 1 1 TRUE
1 0 0 1 FALSE
1 0 1 0 FALSE
0 1 0 1 FALSE
1 0 0 0 FALSE
0 1 0 0 FALSE
0 0 1 0 FALSE
0 0 0 1 FALSE
0 0 0 0 FALSE

Here's the formula that I attempted to use:

=IF(A2=B2, (IF(C2=D2), (IF(A2=C2), "Error3", "OK"), "Error2"), "Error1")

However, I receive an error message stating "The formula you typed contains an error" and it highlights the D in the equation.

I am not sure why the D was flagged as an error. If anything, I thought the "Error" portions would have been flagged...

Is it possible to do a multi-cell validation in Excel?

What was wrong with my formula?

Any help would be appreciated. Thanks for reading!

Respectfully,
Talamon