delete all directories except one

With bash you can do this with the extglob option of shopt.

shopt -s extglob
cd parent
rm -rf !(four)

With a posix shell I think you get to use a loop to do this

for dir in ./parent/*; do
    [ "$dir" = "four" ] && continue
    rm -rf "$dir"
done

or use an array to run rm only once (but it requires arrays or using "$@")

arr=()
for dir in ./parent/*; do
    [ "$dir" = "four" ] && continue
    arr+=("$dir")
done
rm -rf "${arr[@]}"

or

for dir in ./parent/*; do
    [ "$dir" = "four" ] && continue
    set -- "$@" "$dir"
done
rm -rf "$@"

or you get to use find

find ./parent -mindepth 1 -name four -prune -o -exec rm -rf {} \;

or (with find that has -exec + to save on some rm executions)

find ./parent -mindepth 1 -name four -prune -o -exec rm -rf {} +

Oh, or assuming the list of directories isn't too large and unwieldy I suppose you could always use

rm -rf parent/*<ctrl-x>*

then delete the parent/four entry from the command line and hit enter where <ctrl-x>* is readline's default binding for glob-expand-word.