Since my DS did go through the first half of the year in regular kindergarten, I'll just add this....
I think the 4 year olds DO stand out in a kindergarten classroom, mainly by their age. Believe it or not, peer cliques ("grouping" might be a better term at this age) are already starting to appear at kindergarten, and they are well aware of how old everyone else is. Here in Cal, the state lets you start at 4y8m (Dec cutoff), and it is pretty obvious who the 4 year olds are just by watching them walk in the door. I volunteered in the classroom, and I was surprised by how easily I could pick them out.
A large portion of kindergarten is about playing--free play and playground play. Our kids might know much more than the other ones in there, but all they are really going to take from kindergarten is their interactions with peers. If they're left out because they aren't quite at that social level (and I mean more about how they talk/act/games they play than about how well they sit for the teacher), or can't run as fast or kick the ball as far, my experience is that they will feel that in K.
Conversely, DS's entire table wasn't even aware that he was able to write paragraphs, read fluently, and do division problems. One finally asked him 3 months in why he wasn't reading the red (sight word) books. He had been reading from his own book bin in front of them the entire time!
Also, most K teachers that I've talked with won't do a lot of differentiation for the first half of the year. DS was profoundly gifted, and they tested him at the beginning of the school year as 3 years ahead in most areas (2 1/2 in math). Nonetheless, he couldn't get anything but his own book bin for the first few months. They practiced their letters and numbers for MONTHS! DC will get MUCH more writing experience from you at home than from any K program I'm familiar with.
What they will get in K, though, is the fun of being part of a larger school, the excitement that comes from being a Ker, buddy classes with older grades, etc. Also, while my son was going completely crazy with the academic level, two of his very best friends came from that class. They aren't gifted, and they are such blessings in our lives because he has other age-peers that he likes to play with. He's around older kids with all of his academic-type activities, but we are finding that having a summer full of age-peer friends is very very healthy for him at this age.
Just my two cents....
