For those who don’t know: Polyphony actually released a separate disc version of GT4 that enabled online play (Battle Mode, Time Attack leaderboards, and even voice chat via the PS2 headset). The servers were shut down within a year, but the beta disc itself has always been a weird piece of history.
Because the GT4 Online Beta servers are long dead, you cannot play it on original hardware without an exploit. However, the now runs the beta at full speed. Here is how to access the "Normal Downforce" features: