Many developers will develop on one particular platform, and simply test on the other platform later in development. This is quite a practical approach given 4D's great cross-platform compatibility. Maybe you only develop for one platform at the moment, concerned at the cost of buying 4D for the other platform. Yet it is quite practical to test and even do a little debugging on the other platform, without buying a copy of 4D for that "second" platform.
To test the application on the platform you don't have:
1. Use Transporter on the Macintosh to convert the application to the other platform; then
2. Run the application using the free 4D runtime on the other platform and test its behavior.
You will want to test all the functions of your program. You will particularly want to check that your layouts and printouts come out looking okay on the other platform.
You may very rarely run into a platform-specific bug in your code. To track it down, you can download and run 4D in demo mode on the second platform. Since this only limits your ability to add records, methods, tables and layouts beyond a certain number, you can freely run the debugger on code written using your other full version of 4D.
Debugging using the demo version of 4D will often mean that you need to make changes in your full version on the first platform, and transport that over to test the changes. If you find yourself doing a lot of this, you should probably consider at that point buying at least the single-user version of 4D for the second platform.