This could be a third aircraft.
( This ( ( could ( be ( a ( third aircraft ) ) ) ) . ) )
(ROOT (S (NP (DT This)) (VP (MD could) (VP (VB be) (NP (DT a) (JJ third) (NN aircraft)))) (. .)))
The airplane could be the third or fourth.
( ( The airplane ) ( ( could ( be ( the ( ( third or ) fourth ) ) ) ) . ) )
(ROOT (S (NP (DT The) (NN airplane)) (VP (MD could) (VP (VB be) (ADJP (DT the) (JJ third) (CC or) (JJ fourth)))) (. .)))