First, let's determine how many cars Elliot needs to fill all 4 display cases.
Each display case holds 15 cars, so for 4 display cases, he needs:
\[ 4 \text{ cases} \times 15 \text{ cars/case} = 60 \text{ cars} \]
Elliot currently has 28 cars. To find out how many more cars he needs, we subtract the number of cars he has from the total required:
\[ 60 \text{ cars} - 28 \text{ cars} = 32 \text{ cars} \]
Next, we know that Elliot can buy 3 more cars every month. To find out how many months it will take for him to acquire the additional 32 cars, we set up the following equation:
\[ \text{Number of months} = \frac{\text{Number of cars needed}}{\text{Number of cars bought per month}} \]
Substituting in the values we have:
\[ \text{Number of months} = \frac{32 \text{ cars}}{3 \text{ cars/month}} = \frac{32}{3} \approx 10.67 \]
Since he cannot wait a fraction of a month, we round up to the next whole number:
\[ \lceil 10.67 \rceil = 11 \]
Therefore, Elliot will have enough cars to fill 4 display cases in 11 months.