I looked up the number of available new Z4s on cars.com a couple of weeks ago and saw the same thing. There are literally only 14 listed nationwide on cars.com right now. There are over 1000 Supras listed on cars.com right now. Hard to sell in large numbers when supply is virtually zero.I keep seeing this on places like Reddit so let me explain this to you.
The Z4 and Supra are built Magna Steyr - also BMW likely has very low margins on each Z4 sold. Since March of 2020 BMW has pretty much halted/slowed down the production for the Z4 and allowed the Supra to pump up their volume nonstop during this period. How do I know this? I tried to order a Z4 M40i since August of last year and NO dealer in California had allocations for 6 months. California, the state that doesn't rain, cannot get allocations from any dealership (I contacted every single one in SoCal) and they all said that likely won't have allocations until summer of 21. Go to Cars.com now and search nationwide... its been at 0 and the incoming stock are customer cars from last years allocation just arriving and have already been sold.
The reason why Supra sold more than the Z4 is not price or badge related, its entirely BMW refusing to manufacture it. Same thing as trying to order an M8 COUPE last year, impossible.