One of the simpler things to do in SC4 is to create your own custom region layout. All it takes is a 24-bit bitmap of the appropriate size. Each pixel across and down is 1km in size, so the 16km x 16km default size would be 16x16 pixels. Once you have your bitmap, all you have to do is color in the blocks with the appropriate city size color. Small cities are 1km x 1km, and the red channel needs to be set to 255 (you can set blue and green to whatever you want). Medium cities are 2km x 2km and are designated by setting the green channel to 255. Large cities are 4km x 4km and use the blue channel set at 255 as their designator.

Here is an extremely enlarged copy of the config.bmp file that I'm using to create my custom region:



To use this (or your own), you'll want to do the following steps:

1) create the config.bmp file
2) enter SC4, create a new region, then either load a different region or exit SC4
3) copy your config.bmp file into your My Documents \ SimCity 4 \ Regions \ yournewcityname folder.
4) reload your new region and you should see your new tile layout.