I'd go ahead & do it now;
everything's getting ready to go dormant, & it'll wake up next spring in its new home.

If you wait til spring, you may get the wrong one unless you've marked it in some way that won't be displaced when the foliage dies back, & if that happens, you'll have to wait til they all bloom to see which one you want to move.

When you move it, put it right in the ground;
do not soak it like you might soak a rose or some such.

Given too much water, four o'clocks will rot in no time at all.

I have a big pot of them. Will they come back if I leave them out for the winter or should I take them in the garage?

Mine are in the ground, & they come back year after year-
& bring all their children!

but I'm always a little apprehensive about leaving things outdoors in pots, exposed to cold air:

You might pile mulch around the pot or bury it for extra insulation.

I would move the plant before frost kills back the top. I doubt it will care since it is about to go dormant for the winter. I've got four o'clocks in my back yard and they have survived everything you can throw at them; drought, floods, hot and cold weather. The seeds seem to find their way into everything. Each spring I pull them out of almost every pot plant I have. I've dug up both seedlings and older plants and while the top may wilt for a few days, they never seem to die. The tubers made it through the winter of 1988 when it was in the 20's for a full week. Came back strong as ever in the spring.

This year during all the rain, some friends wanted a few for their yard. So we just yanked them up. The wet soil made it easy. The plants went three ways and not a one died. I don't think I would worry about this plant at all.

As far as my experience goes, they are not frost hardy but as alluded to above, they are PROLIFIC seeders. If you have some unusual color, I'd protect it well because it may suffer freeze kill and it may not come true from seed.

They're perennials;
the plant returns each year from the tuber (looks something like a cross between a small sweet potato & a turnip).

The foliage dies back but the tuber sends up new foliage in the spring.

(In fact, mine are beginning to "let go" right now;
if I grab the main stem, it may come off in my hands.)

Four o'clocks do self-seed prolifically, but the original plant will return.

Thanks. I have been toting these around with me in a pot for 5 years. I finally have a house that is mine and want to plant them next spring but I don't have the bed ready yet. I guess I'll just put them in the garage for now. I thought they would return but not sure about leaving them outside. Thanks for all the replies. Do you grow yours in full sun here? I had my original plants around a tree in Lousisiana but they got tall and leggy.