( walking ( on ( all ( four limbs ) ) ) )
(ROOT (S (VP (VBG walking) (PP (IN on) (NP (DT all) (CD four) (NNS limbs))))))
walking on all four limbs
(ROOT (S (NP (DT All) (NNS mammals)) (VP (VBP have) (NP (CD four) (NNS limbs))) (. .)))