jmc

We can line up the numbers and add just as we do in base 10. For example, when we sum the digits in the rightmost column, we get a sum of 4. Since 4 divided by 2 gives a quotient of 2 and a remainder of 0, we leave 0 as the rightmost digit of the sum and carry 2 to the next column. Continuing in this way, we find \begin{array}{c@{}c@{}c@{}c@{}c@{}c} & & 1 & 1 & 1 & 1_2 \\ & & & 1 & 1 & 1_2 \\ & & & & 1 & 1_2 \\ & + & & & & 1_2 \\ \cline{2-6} & 1 & 1 & 0 & 1 & 0_2, \\ \end{array}so the sum is $\boxed{11010_2}$.