Download the effect archive from here.
If you don’t have Motion installed, you won’t have a Motion Templates folder in your Movies folder. Download this set of empty folders and add them to the Movies folder of your home directory.
Although the folder is called Motion Templates, you don’t need to have Motion installed on your computer for this effect to work.
If you don’t want to use the clips from the beginning, edit them from the events library onto your timeline using an in-point of your choice. When selecting the clip to use in one of the three image wells, click the clip in the timeline:
There is the option to turn off the 4 up effect and select a shot to display. You can also choose to highlight one of the four shots in the 4 up display.
If you find this free plugin useful, you might like my Alex4D Animation Transitions plugin pack:
There are many more free plugins on my Final Cut Pro X page