My awk is very rusty too. Unfortunately, your version emits a trailing space, which violates the spec. I've remedied that below and done it in the form of a function so that it can be more easily compared with the other languages.

# File     : revwords.awk
# Run with : awk -f revwords.awk </dev/null
END { print reverseWords("  one two   three four  ") }
function reverseWords(s) {
    if ((i = split(s, words)) == 0) return ""
    sret = words[i]
    while (--i > 0) sret = sret " " words[i]
    return sret
}
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