The first strike was the first World Trade Center bombing. It failed. Something different and bigger was necessary.
The second strike was Oklahoma City. It was different and bigger, but it was covered up by an Administration that needed to blame it on right wing extremists. So something even bigger was necessary.
The third strike was TWA 800. It was covered up because the evidence led back to the same people who did Oklahoma City and the first WTC bombing. The truth would have panicked Americans out of the air (It's The Economy, Stupid!) and unraveled the OKC coverup.
The fourth strike was Operation Bojinka. Twelve aircraft were to be blown up. They knew how to do it because they did TWA 800. It failed when the bombers were arrested and the evidence found.
That's when they started planning 9/11.