First, I visited Ubuntu's documentation for compiling software here and performed only the first step, which is
sudo apt-get install build-essential
Then I loaded Synaptic Package Manager and installed all of the following packages and their dependencies:
The above is quite a long list. Installing consumes about a gigabyte or so in free space, if memory serves me correctly. I let one of the larger downloads run overnight while I was asleep. I discovered these prerequisites through trial-and-error. Whenever the compile failed, or in some cases even if it succeeded, I noted the compiler's complaints and tried to satisfy them through Synaptic Package Manager. It is okay to recompile as many times as one desires until the not-found errors are eliminated.cmakelibqt4-devkdelibs5-devlibgmp-devlibqca2-devlibgcrypt4-devlibboost-devdoxygenlibtag1-devkdepimlibs5-devkde-workspace-dev
Next, I followed the instructions on Ktorrent's download page. Note that the above satisfies all prerequisites (qt, kde, libgmp, qca2 and others left unmentioned) for compiling.
I am now using Ktorrent 4.3.1 with KDE 4.9.5 on Linux Mint Nadia. My configuration file was preserved intact during the upgrade. So far, Ktorrent 4.3.1 works well, although I had to get used to the new tab design, which I think may be better, as it may save a few clicks on occasion.
UpdateKtorrent 4.3.1 is, if anything, faster than the earlier version, and I like the new tab feature. I think I improved performance by disabling DHT and reducing the number of network connections to about 45 or so. In the case of network connections, more is not necessarily better. I also use the uBitTorrent protocol exclusively, not allowing TCP connections, because uBitTorrent protocol plays better with other users on the network. I have not noticed any detrimental impact from excluding TCP connections. I don't know for sure whether my ISP frowns upon torrenting, but I don't want to find out. All my connections require encryption, and I often use KDE's proxy, though not always, because I'm not 100% confident in the proxy being up all the time.
Why did Canonical not release this new version of Ktorrent to users of earlier and current versions of Ubuntu? By neglecting to backport new versions of software, Canonical provides incentives for users to upgrade to their latest version, 13.04 Raring, which is fast approaching its release date.