I am confused about the stability of carbanion III and IV in the image, I think III is more unstable because of electron donating effect (+R) of -OCH3 group but my book says III is more stable than IV, please help. In compound III -OCH3 would donate its lone pair and would make the carbanion unstable but no such effect happens in compound IV. -OCH3 in compound III would also have -I effect to stabilise the carbanion but I have read that for -OR group +R effect is more than its -I effect i.e., its electron donating nature is more than its electron withdrawing nature through inductive effect, +R means electron donating by resonance(In -OR, R is any alkyl group)
I think in 3, carbanion is resonance stabilised. While in 4, C=C-C=O are in conjugation, thus will not stabilise carbanion.