Walt Tornow remembers bridge four by the clear fork in the creek on the upstream side. Fork has - you guessed it - 4 letters. You can see the fork if you go a bit past the middle of the bridge and look upstream.
Middle of photo shows the first part of the two-pronged fork
I remember bridge four because I think standing there offers the best unobstructed view of the Acropolis wall. What's that, you ask? I don't have a good photo, so go to the middle of the bridge (where you are going to marvel again at how much the upstream side has silted in by comparing to the visible ledge on the downstream side) and look up (upstream side). That 40-story rock face is called the Acropolis (not sure why, as it doesn't look like the Acropolis to me, but let's go with it) and A-cro-po-lis has 4 syllables. (As a bonus, 'wall' conveniently has 4 letters.) A little bit past this bridge is the creek-side truck from this post, if you need a 'four-on-the-floor' mnemonic reminder!