You can run more than one copy of A4Proxy simultaneously on the same computer. It may be very useful if you run different browsers at the same time (e.g. Internet Explorer and Netscape), or a browser and a download manager, and would like them to use different A4Proxy settings.
You can start two different copies of A4Proxy, configure them to use different Browser Ports (Proxy Options tab) – the ports used for communication with the browser, configure the two browsers correspondingly (127.0.0.1:[Browser Port]) – and now you can use your two browsers with completely different settings of A4Proxy.
The best way to do this is actually make another copy of the A4Proxy folder and start the second copy of a4proxy.exe from there, rather than launching the same physical .exe file second time. This way the two copies will run completely independently from each other and will be able to save their own settings.