You might want to email John at J&C to see if he can get 4x10 for you. It might be available to him but unlisted because he doesn't know there's a market for it. It's worth a shot...J&C have always been very responsive to me when I've contacted them.
Jim, as an alternative, you could also alter an 8x10 film holder's dark slide to work with the 1/2 format concept. I know this could be a bother, but there is no reason this will not work, and work well. I have a friend who did this with a 4x5 holder for enlarging and it worked much better than I had expected. There weren't any light leak problems along the center of the film, which I found surprising.
Cutting is a bother, but it may be the only alternative for your film of choice. A decent rotary trimmer with a sharp blade, a stop which is fastened with double back tape to the top and a paper punch to keep the marks oriented on the proper corner would be all you need. I've trimmed film on my paper cutter, but I find it to be very stressful.The edges are not as good as a factory cut and I don't like handling the film as much as is necessary to do this. It ups the odds for damage considerably.
Do you have a mat cutting machine, one where the blade rides on a hold-down bar, lik a C&H or similar type? If so, you can cut film with absolute precision and safety using the straight cutter side (not the 45 degree bevel side) of the mat cutter. The hold-down bar keeps the film from slipping, the sliding blade holder can't scuff the film, and the resulatnt razor slit is just as clean as a factory cut. In fact, the slit is a whole lot better than a standard paper trimmer cut.
Another advantage is that you don't need to thread the film under the hold-down strip on rototrim type trimmers, (where there's a potential for scuffing). Another, little recognized disadvantage of shear slitters, be they traditional paper trimmers, or rotating blade types, is that the unsupported slit edge of the film (the one that is pushed down by the upper cutting edge) has more potential for damage such as emulsion cracking or chipping along the slit edge.
With these tidbits in mind, it would be fairly easy for a mechanically inclined person to make a simple dedicated film slitter that uses a hinged hold-down bar, a film width locating stop, and a simple razor blade holder that slides along the bar to slit the film just as precisely as store-bought film.
One last thought: if you decide roll your own; get a pair of anti-static gloves, the kind used by people who assemble and handle circuit boards. Keeps static and dust bunnies under control.
I slit 8x20 film down from 16x20 and 20x24. No problem.
I use Ultrafine 125 (portrait film) from Photo Warehouse (http://www.photowarehouse.biz/). This film is made in England and develops exactly like FP4. They will not say who makes the film - but you should feel sure that it is of good quality.
Note: You will not get film in a triple box like factory packed 4x5 or 8x10 - but if you order enough sheets you can request to get it sent in a box and double packed in the plastic bags.
Another note if you are enlarging and you use Ilford paper... they have Multigrade paper with the same surface names as Ilford which they will cut what ever size you want.
I use the photowarehouse film for 4x10 - 12x20, it's good stuff. 4x10 is about a dollar a sheet. I order 100 at a time. They will sell you less. I think they have hp5 knock off too if you want a faster film, but I'm not 100% on that. I have used the fp4 for years.
I develop mine in PMK, BTW for Pt and Ag printing.