No bash solution yet, so here it goes (pure bash, no external binaries):

STR="  one   two three four    "
ARR=($STR)
LEN=${#ARR[*]}
LAST=$(($LEN-1))

ARR2=()
I=$LAST
while [[ $I -ge 0 ]]; do
    ARR2[ ${#ARR2[*]} ]=${ARR[$I]}   ## push
    I=$(($I-1))
done

echo ${ARR2[*]}
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