So how does it go up to 1280X400 without supporting anything "in between"?
Actually, there are plenty of modes you can dream up inbetween, depending on which one of your Devs/Monitors drivers you start with and how you choose to tweak your overscan settings. People have used tools like MonED to create drivers for 1024x768 (or close) but they tend to be for specific monitors and even then aren't guarenteed not to harm your monitor :-/
As for the strange resolutions like 1280x400, that's pretty normal for the amiga and goes back to the somewhat strange notion of having separate horizontal and vertical resolutions. Back in the OCS day, you had "lores" which was 320 pixels wide, "hires" which was 640 pixels wide. Either could be displayed as "normal" (200 lines for NTSC, 256 for PAL) or "laced" (400/512 lines, interlaced).
Normal "hires", with it's perculiar thin rectangular pixel aspect was generally used for the desktop, since interlaced modes generally caused peoples eyes to melt.
ECS added "super hires", which doubled the horizontal resolution again. In conjunction with interlace, it gives the same aspect ratio as normal hires but with four times the area visible.
Available graphics cards use chipsets that that have the more familiar square 4:3 aspect ratio.
The good news is that there are plenty of different graphics cards available for your 4000, from old Z2 cards (not recommended), Z3 capable cards (the Picasso IV, though old, is highly regarded) up to PCI cards with a suitable PCI board.
The bad news is that such solutions can end up costing a bit. The PIV is out of production so they tend to fetch pretty high prices on eBay. You can get compatible PCI cards for a lot less but then you have to get the PCI adapter itself and they can go for a bit.
