There are two major steps.
Part one of two
1. Locate and go to the directory where you have installed SC4, usually it is C:\Program Files\Maxis\SimCity 4 Deluxe
2. Within this directory, you will see a folder name English, it should contain a file called SimCityLocale, if it does, you are looking at the correct folder.
3. Now rename this folder from English to UKEnglsh (not a typo!)
Part two of two
4. From your START menu, select Run.
5. Type in regedit into the field and click OK.
6. On the left hand directory tree, expand HKEY_LOCAL_MACHINE > SOFTWARE > Maxis > SimCity 4 > 1.0
7. Change the Language value by right clicking it and selecting Modify.
8. Leave it as Hexadecimal and enter 13 into the Value Data field. Click OK.
With these two major steps complete, your sims will now start driving on the Left! If you wish to switch from the Left to the Right - rename the folder to English in step 3, and enter 1 into the Value Data field in step 8 instead.