1. **State the problem:** Simplify the expression $+15 - (-4)$. 2. **Recall the rule:** Subtracting a negative number is the same as adding its positive counterpart. So, $a - (-b) = a + b$. 3. **Apply the rule:** Replace $-(-4)$ with $+4$, so the expression becomes $15 + 4$. 4. **Perform the addition:** $15 + 4 = 19$. 5. **Final answer:** The simplified value of $+15 - (-4)$ is $19$.