4-Hydroxypentan-2-one, $\mathrm{CH}{}_3\mathrm{COCH}{}_2\mathrm{CH}(\mathrm{OH})\mathrm{CH}{}_3$, is warmed with dilute $\mathrm{NaOH}$. The main product by E1cB is

The alpha-hydrogens at C-3 are acidic, so the enolate forms and expels hydroxide from C-4. The product is the conjugated enone $\mathrm{CH}{}_3\mathrm{COCH}=\mathrm{CHCH}{}_3$, pent-3-en-2-one. Pent-4-en-2-one would be a non-conjugated, non-acidity-controlled product.