Plank for 20 seconds then rest for 10 repeat 8 times
...which really doesn't make sense if you can already hold a plank for 4 minutes straight without bothering with the rest periods.
There's really nothing you can do to encorporate planks into the Tabata protocol. The intent of the Tabata protocol is to go all-out (or close to it) for those 20 seconds. There's no way to go all-out on a static hold exercise. There's no sense of scalability. Sure, you can do planks for 20 on/10 off/8 rounds, but that doesn't make it the Tabata protocol.