Yeah, with a 5 foot animal you should assume that it can reach any point in a 4 foot cage from any other point in the cage unless you at least use a shield. And don't underestimate the power of a snake that size to push past a shield either.

Also, think long and hard about how dificult a thorough cleaning of a cage of those dimensions will be. Unless it comes apart, you will be crawling inside, which will be no fun if it's 18 inches high. Anything much more than 2 feet deep becomes a real chore to clean... It might be managable if it opens at BOTH ends, but I bet you'll still hate it.
-----
What goes up must come down...unless it exceeds escape velocity.