Here's one that I've used in the past:
5' w/u
20' @ 95-100% of current functional threshold power
5' easy (level 1)
2 x (5' @ 115-120%@ of current functional threshold power, 2.5' easy)
4 x (0.5' @ 150-175% of current functional threshold power, 2' easy)
5' w/d
I think of this as my "90-90-90 maintenance workout", since the power outputs during each effort are ~90% of my in-season best for that duration. I like it because it is challenging enough to enable me to maintain some semblance of fitness through the winter (when done 2x/wk plus weekend riding), but not so hard that I ever dread doing it.
Now if you're specifically looking for level 4 sessions, here's one that you might wish to try, if only to add a little variety:
5' w/u
8 x (5' @ 105% of current functional threshold power, 1' easy)
5' (or 10', if you're compulsive about filling up a full hour, like I tend to be
) w/d
The above is especially good if you're a TTer/triathlete, as you can do the 5' efforts in the aero position, then sit up for some crotch relief during the 1' breaks.