No, you are not being scammed. Keep in mind that 4,000,000 pesos is only about $300,000us @ the recent average exchange rate of 13/1). So $300k liability coverage is an "adequate" amount. Keep reading to see why it easily might not be nearly enough.
The financial liability for a single human* fatality in mexico is 5,000 days x minimum wage of the state in which the death occurred plus funeral expenses. This total can then be increased up to 5 times depending upon the rules of the state. The total varies radically from state to state (low of about $45,000 with the highest approx $540,000).
See this chart which appears to be up-to-date:
http://www.mexadventure.com/mexicani...fm#death_chart
These are the liabilities
per fatality. So, if you took out a bus or a pickup with 8 workers in the back, start multiplying.
But these are only the criminal and civil penalties imposed by the government.
You will still have your legal bills, possibly (probably?) the legal bills of the estate of the person(s) you killed, property damage, loss of use/income for the rental vehicle you crashed, your travel expenses, oh, and yours and anyone else's medical expenses.
$300,000 (or $4,000,000 pesos) really is only adequate. Especially if they think you are a rich gringo tourist.....
* keep in mind JDivers explanation that the 90-year old three-legged burro you killed was able to talk and was about to go before the UN Assembly to provide the answer to life, the universe, and everything. Thats gonna cost a fair amount, too.