Yeah, Excel 5 is the cutoff. Excel 4 or earlier aren't forward-compatible.
Word is similar, I believe the cutoff there is 5.1. One of the big problems is that for many versions, the Mac versions used completely different file formats than the Windows versions. I believe they didn't truly become the same until Office '97/Windows and Office '98/Mac. Office:Mac has retained compatibility with SOME older Mac versions, but not all, just as Office:Windows has retained some compatibility with older Windows versions, but not all.
Even worse is Microsoft Works. Horribly incompatible with just about everything, and changes its own internal formats every few versions, with no backward compatibility.
Word is similar, I believe the cutoff there is 5.1. One of the big problems is that for many versions, the Mac versions used completely different file formats than the Windows versions. I believe they didn't truly become the same until Office '97/Windows and Office '98/Mac. Office:Mac has retained compatibility with SOME older Mac versions, but not all, just as Office:Windows has retained some compatibility with older Windows versions, but not all.
Even worse is Microsoft Works. Horribly incompatible with just about everything, and changes its own internal formats every few versions, with no backward compatibility.