I've seen problems with this in the past, but I believe it depends on what features you are using and how "out of date" the client is. Basically, only if you test on a machine with Excel 4 can you say it works with Excel 4. Then say you don't support anything before that. That's the best policy.

If you're really stuck, and can't get a copy of an out-of-date copy of Excel, then you could try to locate a client that has one and use them as a beta tester. Not as controlled as in-house, but sometimes more realistic.