I've not lived in zone 4, so I was hoping somebody more familiar with your climate would say something. I have lived in zone 5, where these plants are usually considered, sold, and grown as perennials. They form larger patches each year. Your dianthus should drop seeds around the area, or you can collect them to put where you want. If you use mulch, this is a plant that can use room to grow, like creeping phlox, so I always left a mulch-free ring around them of 2-4".

I've not heard of biennials synchronizing, and many biennial plants live more than 2 years.

Sweet William is definitely a biennial and will do well if you plant for "this year" and also for "next year".
To have continuous bloom you need to have plants started in alternate years.
Some dianthus us perennial and do form larger clumps....but not D.barbatus.
Linda C

This seems to be a matter of regional differences or just different experiences of another type.

D. barbatus at MOBOT.

USDA plants profile for D. barbatus.

Wikipedia entry.

Sold as perennial at plant delights.

If you read further on the description on the Plant Delights page, you will see that this is a fairly new ( 10 years!!) hybrid...

This PDN introduction came to us around 1990 from former garden curator James Stevenson. Unlike most biennial sweet williams, our original clump is now over 15 years old. With so many folks asking about it, we finally got busy propagating and officially introduced it in 2001. Dianthus 'Heart Attack' starts flowering in early spring and continues for months with brilliant red-black, carnation-like hummingbird acclaimed flowers atop glossy, dark green, strap-like foliage. Unlike other biennial Dianthus barbatus that we have grown, this one keeps returning and getting better each year. >>

Dave's garden also supports my statements
And if you read about culture on the Kemper site it says it's a short lived perennial best grown as a biennial,
Wiki says short lived perennial which blooms in the 2nd year.
apparently, though there is one cultivar that is truly perennial.
Linda C
I tried to link to a garden site who's owner's name is short for David....but the GW won't allow links to that site...but he also supports my information

Here is a link that might be useful: fine gardening