Are you happening to be using this:
Important: When using Mini DisplayPort-to-singlelink DVI
adapters, quad output is divided into two pairs (displays 1 and
2, and displays 3 and 4). The displays used in each pair must
be identical (with the same manufacturer and model number)
and must be identically confi gured (with the same resolution).
The pairs do not have to be identical to each other, however