I've been looking at the Speed/Crown Graphic cameras lately but started thinking, other than having something to walk around with that few have or have even seen before, other than TV-will it give me any better quality than the 6x7 Pentax? Many times I've read the "6x7 approaches 4x5 quality".
To answer your specific question, I don't believe so.
I very recently went walk about with my Razzle which is a Polaroid camera converted to take 4x5" film with either a single double dark slide holder holding two sheets, or my preferred back, a Grafmatic loaded with six sheets of film. I have had my Razzle conversion fitted with a 150 lens, which is more or less a standard lens. I can focus down to about 1.5m for portrait work, which I have done a bit with this camera.
The real difference with your current 6x7 Pentax and my Razzle or anything else similar you are thinking of, is the ability to change lenses and still see what the film is seeing via the reflex viewfinder. My Razzle is a rangefinder, accurate as anything else, but nonetheless still a rangefinder. Winner Pentax.
The ability to very quickly run off 4, 5, or 20 exposures is easily doable with roll film, quite a bit more fiddly with sheet film. Winner Pentax.
Weight and bulk. Nominal winner Pentax. It is possible to travel very light with a 4x5 rangefinder, but generally..........................
Outdoor portrait work with a tripod, no winner, unless you have a 4x5 with generous bellows which does enable you to get super close with interesting effects possible. Winner would then be 4x5. Just don't forget bellows extension factor, everyone does at some point in time.
If you disregard the portability factor a little and get yourself a wooden folder that takes a standard lens when folded up, you do have great possibility of swing for effect, not to mention rise and fall of front and rear standards to include or exclude whatever. Winner 4x5.
Studio or controlled shooting environment with a very sturdy tripod. A Grafmatic back that holds six sheets is fantastic, but no faster than a double dark slide holder. However you generally cannot tell your subject (if the subject is live that is) to hold while you wind on then take another exposure, although.....................
Go out and shoot something then realise you stuffed the exposure up when you get home, the ability to develop each exposure one sheet at a time is nice. Winner 4x5.
Looking at your pictures on apug, it would seem you have a reasonable percentage of controlled portraiture type of work. Winner neither, I consider this a draw.
One aspect not available with what your question asked is rear standard focusing. Not possible with Pentax, but possible with many wooden folder 4x5 units and most certainly with most if not all, monorail units. Winner not asked for 4x5.
Over the years I have slowly moved to 4x5 for personal work, this was something that didn't happen overnight. I used 4x5 and 8x10 in a studio environment doing product photography, stopped that about 27 years ago now. Switched back to mainly 35mm (Nikon bodies) with forays to folding 645 and fixed 645 Fuji cameras. Then had the use of an RB 6x7 for a few years, but kept coming back to 35mm. I picked up a 4x5 monorail about 15 years ago but didn't use it too much, then I picked up another 4x5 monorail in better condition and a great 215mm lens, I was off with 4x5.
Then I picked up a Shen Hao 4x5 wooden folder, best thing since sliced bread. This unit combined with a Fujinon 150 that can be kept inside the folded camera is brilliant, but one canot walk around with it hand held.
Then I picked up the Razzle, actually had it personally made with a lens I procured for the conversion. It also has a Fujinon 150 lens, identical to the first one I had. This has been a great thing to walk around with and take whatever I wish, with obvious limits on film carry capacity.
I picked up another 4x5 monorail as a spare for the first monorail for $50, another great buy. Now my cheapy monorail is great.
Early this year I stumbled upon a Toyo 45G 4x5 monorail, bought it within 2 minutes of seeing it, this has been a great thing and is now used for controlled portraiture with a 400mm telephoto lens. The ability to use this camera (as well as the Shen Hao) with focusing via the rear standard for minimal change of the image area, is something to behold, especially when really cose focusing.
I spent reasonable money for the Shen Hao (second hand) and have just spent almost 3 months on the road with this camera, loved it. Had this for about a decade now, still love it.
The Razzle was also reasonable money, but cheap in the context that I got it made specifically for me.
The first two monorails were cheap and quite cheap.
The Toyo 45G was cheap, and I don't mean maybe.
I have a very cheaply purchased super wide angle Fujinon 65mm lens with a Heliopan centre filter (bought the filter after using the 65 once then doing some fall off testing) This would be a great lens for a walk around camera. Very wide angle though. Just covers 4x5, so movements are not flash. Once I picked up the Centre filter this lens is really a go to lens for many applications, not sure about portraiture, though I will give it a go.
I have a cheap wide angle Schneider 90mm Angulon, nice, effective and great.
Then there are my two standard(ish) Fujinon 150 lenses. Brilliant coverage so all the rise and fall as well as side movements. Both really cheap, both very, very nice performers.
Cheap Fujinon 210 very nice lens with brilliant coverage, so squillions of movement if required. Very nice performer.
Not so cheap Schneider Tele Arton 250 this was bought for the Shen Hao for portraiture work, limited coverage but it works well close up in portraiture and great out in the field for pulling stuff in.
Cheap Komura 400 telephoto, this has been a sleeper of a lens. It pulls in subjects from far away on the Shen Hoa and makes it possible with the short bellows on that camera to have this drawing power. Placed on the Toyo 45G this lens is stunning close up with elongated bellows for portraiture.
To re-cap. I believe you are better off with your Pentax.
However, fiddling with 4x5 is really great and really cheap these days. Sort of like driving a very old automobile, you can flog the guts out of it and have tremendous fun yet still stay inside the limits set by the law.
One last comment, all the lenses I have for 4x5 are able to be mounted onto any of my 4x5 cameras (except the Razzle) this is the icing on the cake.
Mick.