"Mary Lancaster" wrote...
I hope someone can help me with this. I have four cells A1:A4. If all four
cells contain data or are blank then thats okay. But if 1, 2 or 3 cells
contain data then I want a warning that all cells must be blank or contain
data.

More alternatives,

=IF(ABS(COUNTA(A1:A4)-2)<2,"A1:A4 partially filled","")

=IF(AND(COUNTIF(A1:A4,{"=";"<"})<4),"A1:A4 partially filled","")