Here's your sanity check:
In short, what this really amounts to is 'what happens when one group of people believes they are better than another group of people?"
Have you never heard a four year old group of kid shouting 'no boys allowed!" or 'no girls allowed'? Kids do understand exclusion, even at that age, whether race is involved or not. So, to introduce race is not a huge jump for kids.
As a preschool teacher, I used a book by Katie Kissinger in my curriculum every year; it's called "All the Colors We Are" and discusses race through examining *how* individuals got their skin color (through their parents, their ancestors, and the places they lived--which has largely to do with the melanin their bodies did or did not produce). We discover through this study that NONE of us are exactly the same and learn to value our uniqueness.
That said, I do believe that we should be open in discussing race with our kids. When my son was about 3.5 years or so, he met one of my husband's African-American coworkers and when the gentleman went to shake my son's hand, Kiddo whispered "chocolate hand" in amazement.
We do have other friends who are black/African-American, but for some reason, this moment made a big impression on him. Kids do notice differences--it's just that young children don't usually actively discriminate against children of other races, so it does appear to adults that "kids don't see color".
In the book "Nurtureshock", authors Po Bronson and Ashley Merryman explicitly combat the notion that kids don't need guidance when it comes to race. In fact, their research showed that parents who didn't speak to their kids about race and give positive messages about people of other races had kids who were MORE likely to think in stereotypes and racist ways than parents who did discuss it and actively encouraged their kids to enjoy friends from races other than their own. Granted, we don't all always have these opportunities, but to say that kids don't see difference is incorrect, from my point of view. They don't always discuss it with us (and sometimes, that's because we don't open up those conversations), however, I am sure that my preschoolers noticed that skin would be different colors just in the same way they brought up eye color, hair color (who had which ones) or who was wearing the neat tee shirt or dress.
My son will likely be discussing Dr. Martin Luther King,Jr at school this week. We've mentioned him several times; our city has a street named after him, so it's a no-brainer for us. Just like the book 'The Sneetches' by Dr. Suess, we've talked about how it used to be that white people were considered to be 'better' than people with dark skin and how wrong that was. One of his older friends introduced him to the story of Rosa Parks and how she stood up to silly ideas regarding race by sitting down, and so we've discussed that. By not discussing it, I believe we shroud our experience of race in shame. We have also briefly touched on slavery and servants, without a lot of heavy-duty drama or going into a lot of detail-- and this was in answer to his direct questions.
For me, what is 'age appropriate' is what my son asks me about. We watch nature videos which show peoples from around the world, so that's another way to introduce a more 'global' perspective of humanity.
by the way: I grew up in Honolulu, the only white kid in my extended family there (my mom's second husband, my adoptive father, and his family are all Filipino)... believe me, there was an awareness, even if it wasn't openly discussed. But even at four, kindergarten kids teased me for being white... kids DO see color. And you'd better believe my son knows that two grandpas are have light skin and one grandpa is brown, as is his auntie (my half-sister). It's just who our family is.