This is from a Denon-receiver non-guru...

3802 is 2 generations older. It was replaced by 3803 which too will soon be replaced based on rumours in the market.

1804 is current generation but lower-end model and provides you 90 watts x 6 at 8 ohms

3802 offers 110 watts x 6 at 8 ohms. See this 3802 review

Which is better - I cant tell you since I haven't heard both.

Both these receivers do not have YPAO / MCACC (Room Parametric Eq feature) If you are going to spend USD 600, you might as well get the latest Yamaha RX-V1400 which has this very useful feature. I paid USD 622 for the RX-V1400 and it has the latest DPLIIx as well.

Good luck in your hunt !