One thing I noticed as well when making terrains - be careful with roads around mountains. Look for anything that looks like a reinforced road going up a mountain (kind of like a "road bridge") because that part of the road will disappear when you import the terrain into sims. Also, it will not render tunnels. Watch for any cross roads near the base of a bridge. What looks ok in SC4 will wind up having the cross road go thru the bridge in sims.
If you want to use the bridge that shows in the sims neighborhood terrain decorations, in SC4 the water level has to be barely noticeable. If the water is too deep, in sims the banks of the river will be too far apart and/or the height of the bridge will not match up with your roads. Also be sure that the terrain level is the same height on both sides of the river.
In SC4, if you lay two roads right next to each other, when you import the terrain, it will make a strip of land between the two roads. That strip of land is wide enought for you to place a 3x1 lot on it. Look towards the upper right to see what I'm talking. I use these as "parkways". In SC4, this was two roads adjacent to one another.