when i installed rockbox on my ipod mini 4g it came up with this message:

bad checksum
It sounds like you may have tried to install a build for a second generation Mini on a first generation one or vice versa. The "1g" and "2g" you can find in rockbox documentations are referring to 1st generation or 2nd generation, not storage which is what I think you mean with "4g" (that would have been referred to as "4GB" but it's not important for the install process). The main difference to distinguish the two generations, I
read about, is that the 2nd generation has the storage capacity engraved on the back - the 1st generation has not.
Keep trying to reset it and put it into disk mode manually, it can take some patience but never failed so far (as the top sticky thread in this forums tells me).