[Occasionally6]

If we idealize the crankshaft and pistons as having no inertia or friction, the energy requirement reduces to that needed to compress the charge and perform the pumping work. One working cylinder will do that

Since the power stroke (4-stroke ICE) occurs over 180 degrees of crankshaft rotation, out of 720, and assuming even firing, 4 cylinders is sufficient to ensure that at least one cylinder has fired and is releasing energy at any crankshaft position.

A real engine differs in that there is friction and that the pistons and crankshaft do have inertia.

With very low (or idealized to none) crankshaft + flywheel inertia the engine speed will fluctuate throughout the crankshaft rotation as the torque from each cylinder varies with cylinder pressure and crank throw.