It's with 4 Bit Period 2 Rotors that things start to get interesting. It's possible to combine two 2 Bit Rotors within a single object, and a rotor which contains both On and Off states.

Currently there are 56 known 4 Bit Rotors. This list is not complete, and only includes those rotors whose Zones of Influence overlap. Objects which contain two widely separated rotors are possible, but not included here.

P2R4-5 casings
P2R4-5 casings

The 134 casings for two parallel Domino Sparks show the variety of possible casings.

P2R4-36 casings
P2R4-36 casings

Also of interest is the rotor which first appears in the Blinker [3P2.1] and the Bipole [8P2.1].