So if you want to capture da duck O' the day you better get a little closer to the quackage than with the 400mm: A quack of 30cm hight can be captured from 6m away and still fill the frame of your humble APS-C sensored camera. Full-framers have to get a little closer at 4m...
But before I get into example pics, here are the main facts from the catalog:
Size: 90x195mm = large, but 65mm shorter than it's bigger cousin

Weight: 1190g w/o caps and collar = not too heavy for such a big lens and 100g lighter than it's bigger cousin:?
Optics: ?? elements in ? groups = ?? Don't have any information, but assume it's a 6-7 lens construction

Closest focus distance/max.magnification: 1.2m / 1:3 (tested and confirmed)

Filter-thread: 77mm = standard

No IS = very bad

AF (no built in focus motor/HSM), so does not work on D40/x/D60-bodies


Covers full frame/FX or smaller = normal

Comes with an antique looking lens-bag

Price: around 300-400€ used = not dirt-cheap

The front- and rear lens-cap are cr*p (as usual with a Sigma lens)

Distance information is relayed to the camera, so the Nikon body can do all the advanced exposure-related stuff with this lens

Aperture ring = yes, just like a Nikon D-lens, moves in 1/2 stops up to f/32

Built-in lens-shade

Tripod-collar can be removed without dismounting the lens


Limiter switch: Yes, with two positions: "full" or 3m to infinity / 1.2-2m.
Versions:
Unfortunately his lens is discontinued and can no longer e bought new. I haven't seen other versions of this lens yet on eBay, but I suspect, that there is an older version, just like with the 400mm APO macro. Rumour has it, that Sigma only built a small number of this lens, so I assume, that there was no HSM version. (Readers who know better should give me a hint!)
As to lens-mounts: There are at least versions for Nikon and Canon.
Motivation:
Now why should one get the 300/4.0 over the 400/5.6?
- it's shorter (=less intrusive) and lighter
- it has a larger aperture
- it has a shorter focal length
These facts make the 300/4.0 less prone to shake so you can shoot a little longer when the daylight fades.
The larger aperture also gives you better control of dof so cou can isolate your subject better at f/4.0 than at f/5.6
And sometimes 400mm is just too much to frame the shot correctly.
But as with all tele-lenses (that are not tele-zooms) as you're working fom quite a distance it's harder in any case to frame the subject correctly, be it with a 300mm, a 400mm or 500mm lens. "Zooming with your feet" works best at focal lengths below 100mm

So this must be the main reason, why Sigma eliminated those fixed focals from their catalog: people don't want to be restricted by one focal length or carry two or three such lenses with them.
Alternatives:
- The Nikon AF-S 300/4.0: An excellent lens (see review here) but look at the price. Even used you pay 800+ € for it (lowest price new from a reputable dealer in Germany: 1160€!).
- The Sigma HSM 100-300/4.0: One of the best zooms in it's class ( see test there) but at 92x224mm, 1480g quite a monster. Costs 1000€ new and is sold used seldom for less than 550€. You can also read my review of a used copy with less than stellar performance in the corners over there.
- The Nikon AF-S VR 70-300/4.5-5.6 (see Gordon's review): again a very good choice including VRII. And a pretty reasonable investment. But you loose one stop at the long end plus some IQ in the corners (might get worse with FX-body)