FWIW, I've got one of those DaYi 617 backs for my 4x5", and it's great. But because it's an extension back and the light has to get through the 4x5 gate too, it only officially works with 90-150mm lenses (I've use 180mm and get slight vignetting to 6x15 or so). Mostly (98% of shots) I use my 90mm SWD with it.
Or there are 6x12 rollfilm backs, but most of them seem to cost about the same as I paid for my 617 back ($4-500), if you're lucky you might get a "bargain". You can always use a wider lens and crop off the top/bottom to make it 3:1 if you want.
But certainly cheapest of all is 4x5" and just cropping. Split-darkslides are possible, I also do that with my 8x10" occasionally, haven't tried with 4x5" yet.
But you've only got a 4x5" enlarger don't you? Stick with 612 or cropped 4x5", otherwise you'll need to shell out for a 5x7" enlarger for 617s (or scan, which is what I do with mine). I've done the occasional B+W 617 contact print, and they do look great, but I could probably get the same print quality as enlarging and cropping a 6x7, 6x6, or 645. 617 panos deserve to be printed big.
ps, sometimes fleabay gives up good bargains on filters. I got a Schneider Centre Filter IV for my Fujinon SWD (OK, mixing brands, but it works) for €121. It's got 86mm threads lens size and 105mm front, so that's too big even for my 100mm Cokin Z / Lee system, and I can't afford to re-invest in 150mm filters. But then ebay threw up another bargain, a 105mm x4 Orange 041 B+W MRC for only £30 (look at their new price at B+H). So that'll do me for B+W work. Still hanging out for a cheap 105mm Polarisor for my Velvia shots, or I can use my 95mm Heliopan (also €100 at the 'bay) with no centre filter for now...
