Two caveats for this fix:
First, it won't solve every instance of crash-and-burn. If you're suffering from random crashes, it probably won't help. But if you're locking up and crashing only in the trade route screen, this may fix the problem.
Second, because it involves editing one of the game's configuration files, you'll get a big ol' "You Cheated!" message when you complete each scenario, so it's unsuitable for online play.
Short version of the fix:
1. Find the \sierra\caesar iv\data\NewConstants.txt file
2. It's read-only, so you'll need to toggle that off from the file properties
3. Open the file and search for this line:
4. Change bool;1 to bool;0, save, close the file and make it read-only again.
Launch game and enjoy your new trading capability.
Hope this is helpful.