🚀 go-pugleaf

> Is there a document which outlines any changes made to Xt between R3 and R4?

Not a complete one, that I'm aware of.  Chapter 13 of the R4 spec
gives a brief summary of the changes that might impact existing
code.  The only other document that MIT has is the public review
draft change document which was distributed last September.  The
review draft changes were in diff-like form but don't include a few
small revisions that were agreed upon before final release.

If you'd like a copy of the September document, send me private mail;
but I suspect you'd be much better off simply diffing the troff.