I worked at ATD during this period, on all the published Lego games. Lego Racers 2 is obviously what it sounds like. Lego Racers 3 was actually Drome Racers. It was only called Lego Racers 3 for a very brief period, I only ever really knew it as Drome Racers. Lego Racers 4 never existed, it was always known as Drome Racers 2 internally. And yeah, it only ever had a pre-production phase and no code was ever written for it to my knowledge, though we did progress our internal engine tech in that direction which was eventually used in the uncompleted Ion Runner.
I also did the port of Drome Racers from PC to XBOX. This was complete a few months before ATD closed, so I have no idea why it was never released by the publisher.