Where does Transmit 4 store temporary downloaded files?
The opensnoop program is remarkably useful, but to answer the actual question in case somebody is in a quick bind, for Transmit 4 the location of the temporary files is:

/Volumes/&lt;Macintosh HD&gt;/Users/&lt;current user&gt;/Library/Caches/Cleanup At Startup/Transmit/server-to-server-B18AC7A9-81C6-4AAD-8F2C-6CD70E65D7FD/data-file-being-transferred


Transmit appears to clean up after itself within a second or two of the upload completing. The name and path of the folder implies to me that the system might clean it up if Transmit doesn't get the chance.

Update 26th July 2018:
Latest version (5.1.5) appears to store a file at a time in:

/Volumes/&lt;Macintosh HD&gt;/Users/&lt;current user&gt;/Library/Caches/Transmit/[UUID]/&lt;host server&gt;/path/to/file


The files are cleaned up more or less immediately after uploading to the destination server (afaict) but the subdirectories seem to stick around for some period of time (such that I now have ~600 files in the Caches/Transmit directory. Perhaps this is used for some logging or history, etc.