There are a number of free remote file storage programs available that allow you to sync folders across multiple computers. In this guide I show how you can use file sharing to cooperatively play a region in Sim City 4.
I will be using a program called SugarSync. You can use any file sharing program like DropBox, Wuala, Amazon, or hundreds of others. This particular one allows setting the location for shared folders, which makes it easier to use without having to change game settings.

Download and run the free SugarSync:
https://www.sugarsync.com/downloads/


Follow the instructions to signup with a new account. Everyone who will be playing should install and register the program.

Browse to your Sim City Regions directory
(Run-> %USERPROFILE%\My Documents\SimCity 4\Regions)

Right click on the Region you wish to share and choose SugarSync->Share...

You may use an existing Region or download one of the many custom regions available at SImtropolis[community.simtropolis.com].





Choose Private Sharing and enter the e-mail address for friends who will be playing the region with you.

Now that your region has been shared it's time for your friends to connect. They should have received an e-mail notifying them of the share.

Launch SugarSync and go to:
Sharing->Shared with Me
Click the green checkmark to accept.









Navigate to your regions folder
(%USERPROFILE%\My Documents\SimCity 4\Regions)
and select Add.










That's it! From now on after you save your city it will be uploaded to the shared folder and updated for your friends to see. You can invite more friends to play in your region anytime by going to the Sharing tab in SugarSync and selecting your shared region.


Have fun and make sure to upload screenshots of your cities to Steam!

The way SimCity handles it's saves can cause problems and corrupt files when playing a region adjacent with someone else at the same time. When you save your city it also saves all border cities to update demands and Neighbor deals.

If you are playing at the same time as someone else on a border city then saving will cause a previous version file to be updated for their city. If they are saving while downloading the update it could corrupt their city file.

The only way around this currently is to not play a city adjacent to another player or make sure you are not playing at the same time if your city borders another player.