Hi Sarah,
Since both your variable (x) and your exponent (3) are the same for all four terms (aka like terms), you can add them together.
X³ has an invisible coefficient of 1, so you will be adding 1x³+1x³+1x³+1x³.
When adding like terms, the variable and the exponent stay the same. What changes are the coefficients, which are added together following normal rules for addition, so 1x³+1x³+1x³+1x³ = (1+1+1+1)x³ = 4x³
Your final answer is 4x³.