I actually moved my child into Sanislo in the 4th grade and it was wonderful. The whole clique thing wasn\'t that \'big of a deal\' (to quote my child) and my child was welcome by all the kids. She was miffed at my moving her there, but adjusted fairly quickly and has thanked me for it since.
Sanislo does a great deal of teaming between the grade levels, which I love, so I think your child would get to meet the whole 4th grade and feel comfortable within a few weeks. I also think that Sanislo, as with any of the schools you mentioned, takes the time at the beginning of the school year to form bonds between the teachers/aides/tutors/administration and the kids as well as provide structured activities to enable the team building process for the whole class.
As far as cliques are concerned: I have had three kids in 4th grade (one is currently in 4th grade) and my experience is that this is a time when kids naturally change friends, because their interest are changing. They are moving on from the doll and car stage into chapter books and soccer/baseball. So the cliques tend to change throughout the next couple of years.
An added, surprize,bonus of switching in the 4th grade for us was that when my child attended middle school she knew kids from her old elementary school as well as her new one. It\'s that \'West Seattle thing.\' Good luck in your decision.