Does anyone know of a source to find household income by ZIP+4? I have found IRS information on each five digit ZIP, but was hoping to find more precise information by ZIP+4.
First, note that ZIP codes are not geographies (see this other answer).
Also, as Kotebiya notes in a comment, there aren't pseudo-geographies for ZIP+4 and if there were, they would probably represent too small a group of people to allow data sharing without concern for individual privacy.
There may be commercial services that approximate this level of detail, but I am extremely doubtful that any public agency makes it available.