1. **State the problem:** Simplify the expression $-3y + 4 - 7y$. 2. **Identify like terms:** The terms $-3y$ and $-7y$ are like terms because they both contain the variable $y$. 3. **Combine like terms:** Add the coefficients of $y$: $$-3y - 7y = (-3 - 7)y = -10y$$ 4. **Rewrite the expression:** Substitute the combined term back into the expression: $$-10y + 4$$ 5. **Final answer:** The simplified expression is: $$\boxed{-10y + 4}$$