3rd vs 4th edition

[Greyfox]

Just mod it and see if you like the results.

Add a +1 modifier or whatever you feel appropriate if you believe a prepared defense in a channeling terrain should be more effective.

"Crossing a bridge" is not defined under the current rules but you could always say on which side of the river a unit  is when it enters a hex containing a  bridge.  You might also need mods to determine on which side of the river (or on the bridge proper) is a unit that was demoralized while crossing the bridge; it may matter if they are on the far side and need to flee across the bridge and suffer the penalty again.

I am not trying to discourage you, i am all for people adding rules they like to make the games better for them, but it's not a simple task.

Please let us know what worked and what did not work if you mod it!

I remembered that I sent an e-mail about this subject to Doug McNair at avalanche press years ago.  Found it within a couple of minutes. What I pulled from the e-mail is very rough and focused on attacking out of the bridge hex, but might work.  Doug recommended that I write up for daily content; nine years later it still hasn't been done.  On 27 April 2008, I sent Doug this and one other recommendation:

If attacking from a Bridge hex to assault units in adjacent Hexes there should be a negative column shift to the attacker.  Basically you are attacking across a narrow front along an avenue of approach that is both restrictive and is subject to being raked by direct fire.  Could be a special rule that applies to only scenarios where attackers are trying
to secure bridges.  Say a negative one or two column shift to the attacker, and/or first fire to a defender when defending against an attacker assaulting from a bridge hex.

Like I said more focused on the attacker; with a negative column modifier for attacking out of a bridge hex, than to the defender.  Thoughts.