I don't think the fronts look box-like at all (except the blunt end of a 91) and 91110 (and the 'good side' of 91119) when coupled right-way-round to mark 4s is the best-looking thing on the network (except perhaps some steam locos) right now if you ask me. If you ask me the 800/801/802 has more of a looks problem than the IC225 - perhaps 'dated' isn't the right word, but the non-plug doors give a slightly cheap-and-nasty look to them rather than the sleek appearance they would have had if plug doors had been used (I also don't like the shape of the yellow (or black on TPE 802s) - it makes the train look too narrow or something - but that could be fixed with a spot of repainting). The Sophia seats on the IEP trains are also awful, but again that could have been avoided.Yes the only thing “old” is the box-like front, which does look very slightly dated compared to the current designs.
However from an interior point of view there is absolutely nothing wrong with a Mk4. I for one would pick one over an IEP.