"To test this, I installed Linux Mint 15 RC today, which is based on Ubuntu
13.04, and downloaded the 4.0 DEB. On double clicking that, I got a warning
that an older supported version was available in the repo, but after
ignoring that, installation ran OK.

So, to sum up, version 4.0 seems to be fine for aunt Martha to install, on
Mint 15, and Ubuntu 13.04."

Did you use MATE or CINNAMON?  Thanks.