How about RCl -> an alkyl phosphate or similar, hydrolyse and then oxidise as an alternative.
It's a shame you already have the 4-carbon chain in place.
Can you try phthalimide + *1,3-dihalopropane*, then oxidative insertion of CO, and depending on what you get out of that, reduction of the acid / acyl halide / ester ... down to the aldehyde. There are safer alternatives to using neat CO, if that's a concern...?
S