Fcitx has been in Ubuntu offical repo for a long time, but Fcitx newer than 4.2 will first appear in Ubuntu since 12.04.
If you want to install update to date Fcitx and related package, you can try to use ppa.
sudo add-apt-repository ppa:fcitx-team/nightly
sudo add-apt-repository ppa:fcitx-team/stable
to use different repository. You can browse two repositories via ppa webpage to check which version of Fcitx it has.
After you add the repository, you need to use
sudo apt-get update
or simply use your favorite package manager to refresh, search fcitx and install it.