Set the starting tempo for the song as you normally would.
Place the cursor at the point where you want the tempo to start slowing.
Hit Shift C. This brings up a dialog box in which you can select a new tempo (and a new time
signature if you want).
Enter a new BPM.
You can repeat this for each of the four bars where you want the tempo to slow, or you can do the first and last, and check a box that asks if you want to 'gradually slow tempo to next mark' (or something like that.
Finally, where you want normal tempo to resume, do the same, but just enter the starting tempo.