I seriously wouldn't worry about filling in any gaps. Any gaps that they do come across at this stage can probably be filled with a one or two sentence explanation as they go.
As for your 4yo, if she is skipping K to go to Pre-Primary, I can't imagine what a gifted 4yo would miss out on by not attending a year of kindy.
I agree with Kriston - if there are some gaps, that will give them something to do for the first part of the year while they get used to new class. That time will likely pass all too quickly & once again school will seem to move too slowly!
Cheers,
Cass
