Prior to the release of Boris 4.0, the UI was completely overhauled. As part of this GUI renewal project, Preferences have been changed. Many items previously included in Preferences are now in the new palette window that holds six tabs: Vector Trace, Spline Tools, Grids and Guides, Keyframe, Keyframe Generator and Timecode. A few of these are brand new; the others are items that were moved out of the old Preferences dialog window.
The first pref you will likely be looking for is the one that sets the keyframe modes toggled by the keyframing mode button at the bottom of the control window.

Take a quick trip to the Keyframe Palette. To open it, if it’s not already open, go to the menu Window>Show Keyframe Palette

Here you can set your normal keyframe mode to linear and the other mode to constant or any other modes you prefer. Don’t forget to explore the other palettes for the different settings you might need, especially the Vector Trace Palette and the Grids and Guides Palette, where you can enable and disable snapping. The newer UI makes for quick access to settings you often need to change.

– Steve Oakley, Emmy-nominated motion graphics artist, Boris FX user and Practical Illusions founder