KDE4 start menu (when is set to classic style) does not show "most frequently used applictions", and it cannot be set to show "most frequently used applications". Please note that "most frequently used applications" is different from both "most recently used applications" and "favorites".  

Differences from "favorites":

1. "Favorites" is a static list of application the user sets directly.  "Most frequently used applictions" is a dynamic list, where the system analyzes which application how many times was launched from the start menu and shows on the list those ones were launched most times. If the user starts to use a new application more frequently than another one on the list, the new application might replace older entries if launched more times. The user also can specify how many entries should be listed in "most frequently used applications".

2. "Most frequently used applictions" is not a menu entry with subitems, all applications belonging to it are at the first menu level. Favorites is itself a menu item with subitems. That is launching an application from "most frequently used applictions" is as simple as clicking start menu item, moving the pointer over the selected application's name and click it. Starting  an application from Favorites is more complex as one has to navigate into the submenu and it is much easier to loose focus. Launching applications from "most frequently used applictions" is therefore more effective, less error prone, and more straightforward.

Differences from "most recently used applications":

"Most recently used applications" does not care about the use frequency, it replaces the items in the list  every time another application is started. Applications used only occasionally may replace those are used regularly; this a big drawback.

There should be a setting for "most frequently used applications" in KDE3 start menu (at least in classis style).

Reproducible: Always