Software - Excel 2003 (11.6355.6360) SP1

I previously merged 4 rows in 4 seperate columns into 4 cells. That is, I
created one larger cell comprised of 4 rows in each 1 column, of 4 columns,
using Excel's merge feature. I would like to split the merged cells back into
their original 4 rows but the merge functionality of the Alignment tab of the
Format Cells' dialog box is grayed out. I have searched microsoft help and
found an atlernative method for spliting cells - selecting the cell and using
the Merge and Center button on the formatting toolbar - but it too is grayed
out. I am not sure what I have done that would have caused this functionality
to be restricted. Any ideas would be much appreciated. Thanks