It should be II (Resonance)> IV. (No donating groups so stable carboanion)>III (One donating group)> I( 2 donating groups which destabilizes negative charge.
Chad's quiz the answer is IV < I < III < II. Is 4 last because it has no resonance?
In the β-diketone, there are two carbonyl groups for resonance stabilization which also increase the acidity of the inner alpha hydrogens.
In the aldehyde, one alkyl group and one hydrogen are bonded to the carbonyl carbon. Hydrogens have no significant electronic impact, so this conjugate base is more stable than that of a ketone, where two alkyl groups will destabilize the system by donating electrons (a carbon does not want a negative charge).
