As I understand the differences, the 4900 uses the same 10-color inks as the 7900 and 9900, which give you a bit more gamut and saturation in the orange and green ranges than the 8-color system used in the 7890. Whether your particular prints would benefit has been the subject of some discussion here - on most photographic prints, the differences are likely to be minor or non-existent, but if autumn colors are your specialty, the extra gamut might be just what you're looking for.
Some people here have experienced clogging issues with the 4900 - I don't know that this forum constitutes a scientific sample, but it seems that the 4900 might be more susceptible to clogs.
If you've the room for a 7890, there's something to be said for have some room to expand your maximum print size...
