I want to know the answer to the following question:
What is the one exception to the NFL's 4-preseason-game limit?
All the background I'm provided with is that "The league has tacitly forbidden the playing of non-league opponents, with the last interleague game having come in 1972 and the last game against a team other than an NFL team (the all-NFL rookie College All-Stars) was held in 1976.". Is that enough to answer the question?
Answer: no