In practice, IIII sometimes has been accepted usage for 4. For that matter, IIC was sometimes used to mean 98. There's also some question about whether you have to subtract only the next smallest number, as in XCIX for 99, or whether it's okay to just go for IC. Enigma's album MCMXC A.D. could probably just as well have been MXM A.D. in the real world, but proper, prescriptive usage would frown upon that sort of thing.