The biggest flaw of the 04 was that gearing was off by 1 tooth, compared to the other minis. I wish I understood why Tamiya did this. Maybe it's because the "standard" mini FDR had only been used in one model by this point, so there wasn't much history to stand by.
Anyway, the 04 was wickedly fast and agile if conditions were good and you set it up well. In very other case, it was a horrendous pig. And for some reason, the last few I ran chewed through stock gear diffs like a fat kid on a cake.