Nah the DA4400ES has been phased out...
The DA3500ES replaces both the DA3400ES and the DA4400ES. The main difference between the 3500ES and the 3400ES is that the the 3500ES adds a component video output to Zone 2 and adds an L/R stereo output pair to make Zone 3. I guess Sony figures that the zones make the 3500ES replace the 4400ES Of course we all know the 4400ES was a heavier/better built receiver than the 3400ES but that's the way it seems to be.
The DA5500ES seems to do the same for the 5400ES and 6400ES, given the 5500ES is now internet capable like the 6400ES was. This phases out the arguably overpriced 6400. That's not as bad though considering the 5400 and 6400 were nearly the same receiver (like the 2400 and 3400). Only thing is the 5500 US version doesn't have H.A.T.S. like the 6400 had (though the Euro version of the 5500 apparently does have it).