R help - Sep 2007 - how to make an orthogonal contrasts matrix?

Hello, R experts,
I have an 11 levels of a factor A. I am only interested in a constrast  
between first 4 vs rest 7 levels. Is there any command to make an  
orthogonal constrasts matrix given one contrast of interest such as   
7,7,7,7, -4,-4,-4,-4,-4,-4,-4?
Thanks,
Joshua

tmp <- c(7,7,7,7, -4,-4,-4,-4,-4,-4,-4)
tmpf <- factor(letters[1:11])
tmpf
contrasts(tmpf)
contrasts(tmpf) <- tmp
contrasts(tmpf)
zapsmall(crossprod(contrasts(tmpf)))