David,
I'm a big advocate of the 4x10 format. I've been shooting 4x10 off and on for over 11 years. I recently completed Part 2 of a two-part article on the 4x10 format for View Camera magazine. Part 1 of the article covered cameras and film holders and was published in the Sep/Oct 2005 issue. Part 2, covering lenses and film for 4x10 will be in the Nov/Dec issue - which should be out any day now.
The smallest currently available 4x10 is the Alt View 410 WA made by Patrick Alt. It is a non-folding design, so it takes up quite a bit of space in a pack, but it is very light (about 4 lbs.) for a 4x10 camera. It is available new directly from Patrick Alt for $1400. It has a shorter maximum extension and fewer movements than the other current 4x10 models - but that's the trade-off for a lighter weight, less expensive camera.
Keith Canham used to make a 4x10 point-n-shoot box camera with a helical-mounted 90mm Schneider Super Angulon XL lens. This camera is a rigid walnut box with a non-interchangeable lens and no camera movements. It's the smallest, lightest 4x10 camera I am aware of. Keith no longer makes this model and I don't recall ever seeing one for sale on the used market. If this is what you are looking for, Alan Brubaker could probably build you a custom 4x10 box camera around the lens of your choice.
As far as lenses go, there are a TON of lenses that cover 4x10 (basically everything that covers 8x10 and several more that don't). Part 2 of may article contains a table listing 49 current and recent lenses capable of covering the 4x10 format (and that doesn't even include monster lenses like the 210mm Super Angulon and other very large lenses capable of covering 11x14 or large formats). The lenses listed in the table range from 90mm - 450mm. I personally use lenses from 110mm - 450mm with my 4x10 camera. I use my 210mm and 150mm focal lengths the most, but that's a matter of personal preference.
By "backs", I assume you mean 4x10 film holders. This is a very complicated subject. There are several different brands of 4x10 film holders - the problem, is they aren't all the same size. So, brand A holders may or may not fit brand B camera. I covered available 4x10 film holder options in depth in part 1 of my article. If you are just starting out in 4x10 and don't already own a camera or any holders, I recommend buying some of the Canham 4x10 holders. They are, by far, the least expensive ($95 each) and readily available. Then, either buy a camera designed to take the Canham holders, of have the back modifed to accept these holders. This is the least expensive and most practical route to a dedicated 4x10 camera and holders. Patrick Alt's 4x10 camera was designed to use the exquisite (but expensive) Lotus 4x10 holders, but if you buy a camera from him, he will modify the back to use the Canhma holders at the time of purchase.
Hopefully, that's enough information to get you started. There is a lot more information on the various holder and lens options in the articles, along with photos of every 4x10 camera currently on the market. 4x10 is a wonderful format. The cameras and lenses are small enough and light enough to carry in the field, but you still get a nice big panoramic negative/transparency.
Kerry