With introduction of QTS 4.x:
- there are a lot of changes in QNAP's Control Panel, including the fact that "Application Servers" component is gone,
QPKGelements are available for download in the "App Center".
This causes my "Installing Optware (IPKG) on QNAP TS-210" (6,5 year old!) article to be completely outdated.
Since I went through this guide and can confirm that it is working, I will be enough nasty to copy-paste that article here (removing stuff not related to QNAP TS-210 and fixing some bugs).
To go through these steps you need a shell access to your QNAP. Using the PuTTY is the most obvious answer.