RK fourth order method for a 2nd order differential equation

d^2y/dx^2 + 0.6*dy/dx 0.8y = 0

parameters: y(0)=4 and y'(0)=0. from x=0 to x=5 with step size; h =0.5.

I have this 2nd order ODE which I need to solve use RK 4th order method:

But I also need to calculate value of each state variable at a different point of x = 2, using h values

of h=(0.5)^p, where p = [1, 2, 3, 4, 5, 6, 7]. and then plot(on a log-log scale) the relative error for each state variable at x = 2 as a function of h and then use that solution for each state variable at x = 2 using h = (0.5)^20 as the true solution.

I have posted my code for the RK fourth order but confused on how to go about this part. Kindly put me through. Thanks

% Using the RK 4th order to solve equation d^2y/dx^2 + 0.6*dy/dx 0.8y = 0
% parameters: y(0)=4 and y'(0)=0.  from x=0 to x=5 with step size; h =0.5.

h=0.5;

%Initial conditions
x(1)=0;
y(1)=4;
z(1)=0;
N = 10;

for i=1:N
x(i+1)=x(i)+h;
 
k1=h*(z(i));
k11=h*(-0.6*z(i)-8*y(i));
  
k2=h*(z(i)+k11/2);
k12=h*(-0.6*(z(i)+k11/2)-8*(y(i)+k1/2));
  
k3=h*(z(i)+k12/2);
k13=h*(-0.6*(z(i)+k12/2)-8*(y(i)+k2/2));
  
k4=h*(z(i)+k13);
k14=h*(-0.6*(z(i)+k13)-8*(y(i)+k3));
  
y(i+1)=y(i)+(1/6)*(k1+2*k2+2*k3+k4);
z(i+1)=z(i)+(1/6)*(k11+2*k12+2*k13+k14);
end
x;
y;
plot(x,y)