Sindel stays mostly the same as in MK3, while gaining advantage of using combo started from the air combo.
Sindel shoot Fireball through her mouth. She can do it a straight missile or while airborne (causer her to spit it diagonally, thus not an anti-air move. Beside these, she can also use her Wave Dizzy to stun her opponent, which allows her to do any move and combo right after. Lastly, she has a unique move that cause her to Float in the air. In this mode, she can fly around the screen very fast and shoot her Air Fireball. She can exit this mode at any time (or when she gets hit). Sindel has many combos, based on two main one. One of them cause the opponent to be launched in the air, for juggle possibilities.
Against the CPU, Sindel has to mainly rely on her Wave Dizzy or combos started with a jump punch or neutral jump kick. Shooting Fireball or Air Fireball from far often catch the CPU. Her Float mode, however, is probably the move you will never against the CPU, since Sindel can only shoot Fireball and she will quickly get hit by her opponent.
As a side note, a CPU Sindel will never use her Float move. This is one of the very few moves that a CPU will never perform.
TAS tools were used to make this high quality playthrough video.