In my climate, four-o-clocks are not hardy in the ground over winter, and
they take a long time to bloom from seed (as late as late August). One year
I did buy the tubers, and was very happy to get flowers starting in July on
very large handsome plants. I would do it again, but they haven't been
offered since....
A number of years ago I bought a package of roots/tubers labeled
"mirabilis"
at a local garden center. It was attractively packaged, and I didn't
recognize them from the plant picture. When they began blooming, I
realized
I had paid $4.95 for three four o'clock roots when I could have purchased
a
whole package of seeds for a $1.95. It was kind of funny but also taught
me
a lesson. GRIN

Yes, the roots/tubers can be dug, stored in a cool, dry place, and
replanted
the following year. I don't bother now because they can be grown so easily
from seeds and so freely reseed themselves that I consider them a weed.

John