Well... Didn't you say you knew about Motion?
Ok! Start with one still, put it on V2 or higher. Set it's duration to 10 seconds. Right-clik, get in the Motion dialog box. The inverted triangle on the left of the timeline is your keyin point. Now click somewhere on top of the timeline, to create a new key point. With the mouse pressed down, along that timeline, drag this key point to the 5 seconds mark. That will be the lenght of your move, then the still with hold it's position for another 5 seconds.
Now click on the beginning of the timeline again, the triangle will move over your keyin point. Type 50% in the zoom field without hiting enter. The still on the left of the Visible area should now appear smaller. Click on the red center point and drag the still to position it's right top corner just leaning against the left bottom corner of the Visible area. You should now have a little still diagonaly aligned to the Visible area. Click over the second key point you just created, the triangle will move up to that point. Now drag your little still so that it occupies the bottom left portion of the Visible area, aligned with the edges. Take note of the v & h positions in the info box (you could have changed those to fine tune your positioning), and hit the end of the timeline. The triangle will once again follow. Now type the v & h positions you noted and set the zoom value to 50%.
You should now see, in the preview window, your small still starting off-frame moving into position and holding there for 5 seconds... leaving space for your other 3 stills.
For the other stills, just repeat the same process, making sure they are on their own track and changing the position from where you want them to come from.
Willing to try this out up to this point?
B-)