Frequently asked questions - Base
Generally, this is because the button F4 has been assigned, either by you or a macro, to another action, for example, if you have installed the extension "Dmaths" by Didier Dorange-Pattoret.
You can check this by making a right mouse button click on the main toolbar, and choosing Adaptation then the tab Keyboard.
You should then be able to see that F4 corresponds to Insert an OpenOffice.org Math object.
In this case, if you want to access your data source browser, do so via the menu entry Display > Datasource.