The inks on the 4800 are better, except for matte black , which is the same. The 4880 has improved the magenta further, giving extended blue/magenta/red gamut.
The better inks are more linear , have a better gamut and less colour constancy failure. With a light light black highlight tones are smoother, especially in B&W.
The 4880 has other mechanical improvements but we have not seen too many of them yet.
The only real advantage of the 4000 is it can change photo black to matte black without completely flushing out lines.
The 4880 is the go, especially if you don't need to change blacks much. There is not much point getting a 4800 now unless it is real cheap.