Are all of my ancestors from 400 million years ago fish?
Yes they all were because there weren't any land vertebrates, vertebrates were all fish at 400 million years ago. 

It is a close call based on fossil evidence from 395 million years ago, there are footprints of 3 meter tetrapods from littoral/seaside fossils. https://www.sciencedaily.com/releases/2010/01/100107114420.htm

The earliest land vertebrate skeleton is from 375 million years ago. https://en.wikipedia.org/wiki/Tiktaalik



It is technically still a fish, so probably yes, the ancestral species of all land vertebrates were still fish. There are various experimental early quadrupeds, some with 6 fingers, which are cool reading. 

They still exist in the form of lungfish.

Here are images of the earliest land vertebrates