OK. I am assuming you have both Sims 2 and Sim City 4. (You can also do this with Rush Hour, the Expansion Pack for SimCity 4, installed.)
Make sure Sims 2 is closed down and start up SimCity 4.
In the Region View, choose a small sized City Lot. Only the smallest size will work. The larger ones will simply not import into Sims 2.
Sim City 4 has powerful tools for terraforming. Do remember that only the middle section of your map will actually be usable in Sims 2, so any features you want need to be within the central half area. I'm going to call this terrain "Twin Peaks" after a certain cult TV series.
I made two hills using the Hill tool and used the Erode tool to make them look a little more natural.
There should be a river. In SimCity 4, the tools work "smaller" if you zoom in, so I will zoom in to make a small river, as I need to leave plenty of space for building my town.
After a lot of "tweaking" I ended up with this
Now, as this terrain is for Sims 2 and not for SimCity 4, there are a couple more things to do.
For Sims 2, it is much easier to build if the lots are fairly level. Houses on hills are interesting but much harder to do.
So level as much as possible of the terrain while keeping your landscaping features. I use the Soften tool for this as it gives a more natural look. You can also use the quick level tool but that makes things rather too flat for my taste. Experiment till you are happy.
When you are happy with your terrain, save it, just in case it all goes pear-shaped in the next phase.
At this point you could add trees (except palm trees - for some reason they don't work) - but I find it better to add the trees and decorations in Sims 2 as it gives more flexibility within the city. Unless you want to make an inviolable forest area which can never be built on?
Now go into Mayor mode and name your city. Freeze time and add some stuff.
Twin Peaks has quite a large population, so I have put in loads of roads, and there's a bridge over the Falls.
Now save the city in SimCity 4. Exit.
Open Windows Explorer or My Computer and find the file for this terrain - this one is in My Documents\Sim City 4\Regions\Maxisland and is called City - Twin Peaks.sc4.
Copy that file to My Documents\sims2\SC4Terrains. It is now ready to go into your game.
Now go to this page to see how to import the terrain into your game.