I've heard this (loss of 35-50% within 3-4 years) and it's not really true. (Or at least, depends on the vehicle.)

A 2017 Honda Accord goes for $20k new. A 2014 of the same model with 45k miles goes for $16k (KBB). A 2011 of the same model with 90k miles goes for $10k (KBB).

Basically, the first 3 years and 45k miles it depreciates $4000. The second 3 years and 45k it loses $6000 more in value.

Arguably it's cheaper to buy new and sell after 3 years than to buy 3 years old and sell after 3 years.

The years 7-9 are when the car stops losing value so quickly. Worth $6700 in year 9 and 135k miles. The last 3 years and 45k miles only depreciate $3300.