Here you did not specify the input and the there is no equation for the output. So you will find the jacobbian that will provided you with A and B matrix of a linear system. It is assumed you know what your u is so that you can subsitute zero for it and the states which is where you are linearizing your system about.
To get Matrix A, a11 is the derivate of x1dot with respect to to x1. a12 is the derivative of x1dot wrt x2....a14 is the derivateive of x1dot wrt x4. Then a21 is the derivative of x2dot wrt x1.....a24 is the derivative of x2dot wrt x4......a41 is the derivative of x4dot wrt x1....
Do the same above but wrt to the input to get your matrix B elements. Then form the matrix.
MATLAB has the command 'jacobian' for finding the jacobian of scalars and vector functions
