[horde] Impossible to upgrade Webmail to latest version, pear doesnt see the latest release

[Louis-Philippe Allard]

I have been trying to upgrade to the latest webmail version of Horde for
several weeks (if not months) and I cannot.  Basically, according to the
Horde's website instructions to upgrade from 4.x to 5.x, I should only have
to run

***
pear upgrade -a -B horde/webmail
***

But I get:
***
horde/webmail is already installed and is the same as the released version
4.0.9
upgrade failed
***

Almost looks to me like "pear" couldnt see that there is a newest version
of Webmail out there.  From within the admin page of Horde, I click the
"Check for newer versions" button, and immediately it says "A newer version
(5.1.0) exists.".  So I am confused. I tried upgrading php but it didnt
help either.

Can anyone help with this unresolved issue??
Thanks!