Acetone–butanol–ethanol fermentation
Well, I’m not sure I should answer this, as your question is predicated by an incorrect supposition:

Acetoacetyl-CoA is a C3 unit.
3-Hydroxybutyryl-CoA / Butyryl is a C4 unit.

Not according to Kegg:

However, my answer is worthwhile if I explain how I found this out. One way to check this sort of thing is to go to the Kegg website and search for the compounds of interest, find the appropriate pathway (shown below) and then click to check compounds and reactions.

There is also the Metacyc website for metabolic pathways, and you can check the formulae on Wikipedia.