I was four years old at the time.
( I ( ( was ( ( four years ) ( old ( at ( the time ) ) ) ) ) . ) )
(ROOT (S (NP (PRP I)) (VP (VBD was) (ADJP (NP (CD four) (NNS years)) (JJ old) (PP (IN at) (NP (DT the) (NN time))))) (. .)))
I was four years old then. 
( I ( ( ( was ( ( four years ) old ) ) then ) . ) )
(ROOT (S (NP (PRP I)) (VP (VBD was) (ADJP (NP (CD four) (NNS years)) (JJ old)) (ADVP (RB then))) (. .)))