Solving system of differential equations

[milbos]

Hi
I have a system of 4 DE and I have problems with writing syntax to solve it.
Does anyone have any suggestions?
The basic system is like this:

M*x''+Gx'=0

M and G are size 4x4, and vector x is size 4x1.
I know the solution if I have one dif. equation, but I don't know how to solve system of dif. equations.

Thank you for any answer I will get.