4 feet long, as long as there are many leaves would be fine for a cutting. Better yet, why don't you layer the plant to propagate? Nick the stem (don't cut it), dust the nick with hormone and bury that part of the stem (while still part of the actively growing plant). If it goes well you will end up with a rooted clone and it is likely your layering will encourage the original plant to start putting out branches to thicken out.

~Chills

Thanks Chills, I will try that. In fact, you might have just given me a good excuse to try some of that rooting goop Mom bought, the clear stuff that's a growing medium with hormone already in it. Still might end up using plain old potting mix though, as the goop was pricey.

Another way that sometimes works is to bend the stem. Since they like to climb if it is bent it will at times send a shoot to get vertical again. Make sure it is not completely pinched at the bend. I've had racemosa, edulis, and capsularis send out side shoots this way.

Another cool idea; but it seems mother nature beat us all to the punch--I noticed a 1/2 inch or so new shoot just coming up from the dirt Monday when I was setting up for rooting. It must have been forming & just broke the surface that day.
Come to think of it, I'd better make sure that shoot isn't crawling off to anywhere peculiar.

Well, the layering didn't work out. The goop kept drying out, despite my efforts to keep it moist.
But, my incarnata really seems to be enjoying its new pot and summer out in the yard. I gave it a little food recently and it even looks like it may bloom.
Is it just me or does anyone else think the buds look like something off "Sci-Fi" or "Star Trek"?