Subwoofer recommendation for Dynaudio Confidence C4

Look at a pair of 18.0 or 21.0 from Funk Audio. Nathan can match the finish of your C4. 

If you don’t care about finish that much, may want to look at JTR S1. 

Try a JL Audio e110 or e112. Great subs.

I took over a year to decide on my last speaker upgrade, the Dynaudio's were "finalists" in my search, eventually bought a then brand new model, a pair of Focal Sopra No2's.  IMO both these speakers' bass response is good, for as low as they go, but IMO there's no speaker in that price range that can truly compete with a great sub woofer in the low bass region.
I compared the JL Audio F112V2's with the F113V2's and chose the latter.  IMO, the F112V2's might have been a better match for a pair of stand mounts, as they go higher up in the bass range than their larger brother.  But with the bass that both the Dynaudio's and the Focal's produce, either model would contribute a lot to the final sound.
As for going with a pair of subs (that's what I did), there is something "visceral" in the musical experience when a low bass sound is coming predominantly from one channel or another.