Guest Entries - Two ajax forms for different section entries. Receiving 404 only on second form?
I shall answer my own question.

The issue was because I was logged in as admin in another browser tab. I was not receiving the "Unauthorized" response from craft. Just the 404. By testing in an incognito window I was able to see I was receiving an unauthorized response from craft which led me to the problem. Which was the sectionId for that particular form was incorrect. SectionId on second form should have been 2 not 4.

Hope this helps anyone else having a similar problem.