How to compare the rate of decarboxylation? For the following acids compare the rate of decarboxylation on heating
I $\ce{Ph-CO-CH2COOH}$
II $\ce{Ph-CO-COOH}$
III $\ce{CH3-CH2-COOH}$
IV $\ce{HOOC-CH2-COOH}$

I tried this question by forming carbanions and comparing their stability. But I am stuck at comparing I and IV.
First thing to see is to see carbanion stability.

Now the question arises that between  carbonyl and COOH  groups which can stabilize better. For that you can look at the COOH 's already possible resonance,within its molecule, which decreases it electron withdrawal  tendency.
Hence order would be: I>IV>III>II.