A 4-axis simultaneous surface milling feature requires your machine to have four degrees of freedom. For FeatureCAM, the four degrees of freedom are restricted to moving in X, Y, and Z, and a rotation about the stock indexing axis (A, B, or C in the case of turn/mill). You may define 4-axis features in a turn/mill document with a tool that 'cuts from the OD'. You may also use 4-axis features in a 4-axis indexed milling environment in a milling document (see the Indexing tab of the Stock Properties dialog).
In an appropriate document type, with an appropriate post (.cnc file), some surface milling operations have a 4-axis tab. The 4-axis tab enables you to control or specify the tool orientation. The default value is Vertical, which is used for standard 3-axis milling. However, it can also be a continuously changing orientation for 4-axis simultaneous machining. With 4-axis simultaneous, FeatureCAM tries to machine the feature's surfaces while tilting the tool axis in the way you specify.
4-axis in FeatureCAM is the same as 5-axis, (there are no new terms, dialogs, or concepts to understand) with one important exception: during toolpath calculations, all tool axes are projected onto the appropriate plane. For instance, if the current document is an index-around-X type, then at a particular toolpath point, the tool axis is projected onto the STOCK YZ plane. This results in the toolpath point at (XYZ), and a tool axis of (0JK) so that (0JK) can be converted to an A angle in the posting process.