Fourth-dimensional thinking is a concept that refers to the idea of factoring in the existence of obstacles in different time periods.
This concept was implemented in Back to the Future Part III when Emmett Brown told Marty McFly "you're not thinking fourth dimensionally", when Marty feared colliding into objects or falling off edges.
Common knowledge notes that time is the "fourth dimension", and that Marty was supposed to take the Delorean back and fourth in time, hence the term "fourth-dimensional thinking".
- When Marty is told to accelerate 88 m.p.h. in the DeLorean time machine toward an American Indian display at the Pohatchee Drive-In Theater in 1955, but Doc Brown tells him it won't be in the way in 1885.
- When Doc uses a railroad track with an unfinished bridge, and Marty is scared of falling off the cliff, though that bridge would complete construction, and still be safe after decades of service in 1985.