Stuck on laplace transform question

Question

I have to solve the following initial value problem using the laplace transformation:

$$y'' + 4y = 0$$$$y_0 = c_1, y'(0) = c_2$$

I have taken the laplace transform of both sides, then rearranged it, then subbed in $y_0$ and $y'$ but now I'm stuck on the reverse laplace transform bit... any help would be great. Thanks

Where I'm stuck. Getting $y(t)$ from:

$Y(s)=\frac{sc_1+c_2}{4+s^2}$

Answer 1

Hint:

$$Y(s)=\frac{sc_1+c_2}{4+s^2}=\frac{sc_1}{4+s^2}+\frac{c_2}{4+s^2}$$