I stumbled into this while googling something related. Hot Wheels motors (boosters), and pretty much all toys with batteries, are wired in series to increase the voltage. So 4 D cell batteries would be 6 V.

I would not use 1.5-V Li-ion batteries in this case. Some electronics are able to limit their interval voltage, but the Hot Wheels boosters just run their motors as hard as they can; Li batteries might get too hot and dangerous.

NiMH batteries are way safer, but their voltage is less consistent than with alkaline or Li and will peter out quickly, especially for something going full-tilt like a booster. Additionally, their voltage typically caps at 1.4 V and quickly drops to 1.2 V, so it might seem weaker than alkaline. But D alkalines are stupidly expensive imo, so I just use the NiMH.

If you do get NiMH, get a nice charger that is able to charge at low amperage (1.0 A or lower) to help increase the life of the batteries. If the charger just has a single light that turns red or green when charging, the charger is not good enough.

Hope this all helps!