What is the IUPAC name of this carboxylic acid?

propanedioic acid

why would it be 1,4-butanoic acid?  it only has 3 carbons.  Butane has 4.  It also is a di- carboxylic acid right?  Dicarboxylic acids (at least the shorter ones) are usually named by (and IUPAC has adopted) their common names. 
https://en.wikipedia.org/wiki/Dicarboxylic_acid

this particular molecule is usually called "malonic acid" 
https://en.wikipedia.org/wiki/Malonic_acid

First, there are only 3 carbon atoms and not 4, so this cannot be any derivative of butanoic, but it is a derivative of propane. Since there are 2 carboxyl groups, it would be propanedioic acid. And fully, 1,3-propanedioic acid. Its common name is "malonic acid".

propan-di-oic acid. 

NB There are only three carbons in the chain, hence it is propan; NOT Butan;.

NNB There is no need for branch numbers, because acid functional group only exist on the emd of the chains. 

The IUPAC name is malonic acid.