don't worry so much about the 4th grader because at that level every year there is so much review and covering of the same materials again and again that she'll pick up what she's missed.

As for your older one I would probably just call him a 7th grader and leave it at that. Then move forward from there and keep schooling him through the summers until he's caught up to grade level.

There will ALWAYS be gaps because no one curriculum covers everything and kids pick up on some stuff really easily and other stuff just flies right past them.