With Export, I am generating pdf files with pdf version 1.5. Is there a way to generate them with pdf version 1.4 instead?

• There is a file "\SystemFiles\Converters\Binaries\Windows-x86-64\PDF.exe" that be used from PDF creation. Possibly replacing this file with one from an older installation may work. Remember to make back-ups if you try this! – Mr.Wizard Sep 18 '14 at 10:39

AFAIK, the only documented options to Export pdf files are "ImageSize", "ImageResolution" and "AllowRasterization", so my answer would be no, natively.
Run["gs -sDEVICE=pdfwrite -dNOPAUSE -dBATCH -dSAFER -dCompatibilityLevel=1.4 -sOutputFile=output.pdf input.pdf"]

There is currently an open case "CASE:1570212" about metadata in pdf export, and I hope they will work on other Export options too. You could write to support@wolfram.com requesting that feature.