How to estimate settling time of an overdamped system?
Yes, your inverse Laplace calculation is correct.
The final steady state value will be 5/8 - this is the DC value after a long length of time. So, you are really looking for the rest of the equation to fall in magnitude to 2% of 5/8: -
$$\dfrac{5}{8}e^{-4t} - \dfrac{5}{4}e^{-2t} = \dfrac{5}{8}\cdot \text{0.02}$$
$$=\dfrac{8}{8}e^{-4t} - \dfrac{8}{4}e^{-2t} = \dfrac{8}{8}\cdot \text{0.02}$$
$$= e^{-4t} - 2e^{-2t} = 0.02$$
Does that help?