Tricky, I think your proposed synthesis will not work. In order to make a compound like [4] (in the synthesis of pHarmacist), we need to prepare a 2-benzylpropionic acid instead of a 2-phenylpropionic acid.
There is a significant difference in the reactivity of Ar-CH2CN and Ar-CH2CH2CN
If you look at the proposed reaction mechanism, you see that the base (K2CO3) abstracts a proton from the benzylcyanide, and the resulting carbanion reacts with the dimethylcarbonate. The carbanion is stabilized by 2 electron sucking groups, hence it can easily be generated by a weak base.
With Ar-CH2CH2CN, it's a whole different story.
First, you will need a far stonger base to generate the carbanion since the carbanion is only stabilized by one electon sucking group (K2CO3 will not work).
And second, the place of proton abstraction is ambivalent. Which proton will be abstracted? The one from the methylene next to the aryl group, or the one from the methylene next to the cyano group. I think both reactions will take place, especially if the base is strong enough. You would need information about pKa of the protons on the methylene groups of Ar-CH2CH2CN, in order to devise a selective reaction (chose the right base).
Third, it is likely that the required stong base will react with the dimethyl carbonate.
