They are similar but according to some reports, the Mits hc4000 actually has higher Native On/Off, but the hc7800 has higher contrast overall due to the IRIS (but some hated the IRIS on the hc7800).
The hc7800 might be slightly sharper, but overall in 2D they should be similar other than the IRIS difference and the fact that the hc7800 also has frame interpolation (because it is 120hz vs 60hz on the hc4000). Oh and the hc7800 will be a bit brighter.
If you are not into 3D, I'd probably save the money and just get the hc4000, but if your main area of interest is contrast and black levels then I might look somewhere else entirely (just depends). However, if going purely by PQ and not by black levels, you can't really lose on the hc4000 or hc7800 for 2D is my thinking.