1. **State the problem:** Simplify the expression $$-\frac{1}{2}a + \frac{3}{2}a - 4$$. 2. **Identify like terms:** The terms $$-\frac{1}{2}a$$ and $$\frac{3}{2}a$$ are like terms because they both contain the variable $$a$$. 3. **Add the like terms:** $$-\frac{1}{2}a + \frac{3}{2}a = \left(-\frac{1}{2} + \frac{3}{2}\right)a = \frac{2}{2}a = 1a = a$$ 4. **Rewrite the expression:** $$a - 4$$ 5. **Final answer:** The simplified expression is $$a - 4$$.