I would say that 4a/4b at the end of Year 4 is a bright child, but not unusual for the year group. (Both of mine were at that level, they are in no way genius's). If she did skip a year and then progress on to Secondary in a year, it would be unlikely that she would be in top sets from those levels (round here anyway).
Lack of a gifted and talented register is no real marker of not teaching a child to their ability, it is the differentiation within the classroom that matters more than occasional 'bolt-on' activities. My DS's primary does not have a G&T register, but DS2 was already hitting 5c/5b by Christmas in Year 5, and during DS1's Year 6, they encouraged him to start a science club for the infants to extend him in that area. Your DD has been differentiated for this year; have you been told that she won't be next? Remember that level 6 is now available in Year 6, so when she gets there it may not be just the standard Year 6 SATs she will sit.
Two sub-levels in a Year is not concerning; three sub-levels over two years is expected progress, so five over two years is clearly very good. Remember that children don't always learn in a linear fashion either - there will be times when they make great leaps, and times when they consolidate their understanding before making the next great leap.
Finally, although your daughter is currently mature, puberty changes things - plus there is the logical conclusion that if you are a year ahead at school you will leave home for university a year early, and there is no way to judge now if she (or you) will be ready for that when she is 17.