Squeak,
I have a partial answer for you. I haven't heard back from Yamaha yet but when I came in this morning I experimented with the keyboard. You are correct that when you clear all sections, the default of 2 measures comes back even if you selected a style which uses 4 measures. However, what i found out in playing with the keyboard is that by only clearing one section (main A) all tracks, I still have use of all 4 measures. I don't know if this is the only way, but it is a way to be able to record 4 measures, all parts without going back to 2 measures.
When I hear back from Yamaha I will confirm if this is the only way to do this.
George Kaye
Kaye's Music Scene
Reseda, California