Final answer:
David will earn $3,300 per month in 4 years.
Step-by-step explanation:
To calculate David's monthly earnings in 4 years, we need to determine the total raise he will receive over the 4 years and add it to his current salary.
Given that David gets a $50 raise per month, the total raise he will receive in 4 years is $50 x 12 months x 4 years = $2,400.
Adding this raise to David's current salary, his total monthly earnings in 4 years will be $900 + $2,400 = $3,300.