Crap, really? I thought 4000 would be better than 3600 or 3700.
The X series of avrs from lower to upper goes from the 1000 series, 2000, 3000, 4000, 6000, 7000, 8000 so far (but no current 7000 series, altho the 7200 in its time was the flagship model). Within each model range, the second digit differentiates it from previous, your 4000 was the first of that series, it's currently on 4700. I've got a Denon avr from around that time that was the flagship that year and has the odd model number of 4520. Current flagship is the 8500. They used to have different 2xxx, 3xxx, 4xxx and 5xxx series, too
, I've got an older 3808 from that range.
With your X4000 you might have some limitations compared to more modern gear on the 4k aspects IIRC. Definitely a more capable avr than yout 500 was, tho.