jmc

The number of people mowing and the time required to mow are inversely proportional. Letting $n$ be the number of people and $t$ be the amount of time, we have $nt=(4)(6)=24$ because 4 people can mow a lawn in 6 hours. If $m$ people can mow the lawn in 4 hours, then we must have $m(4)=24$, so $m=6$. Therefore, we need $6-4=\boxed{2}$ more people to complete the job in 4 hours.