Multiple errors. When you call your function handle f, it is assumed that the P and Q are values passed at time t. They are not functions or function handles. So just use P and Q, not P(t) and Q(t):
f=@(t,P,Q) (-P/(R*C))+(Q/C);
Then in your RK4 code, the k's are not part of the t calculations. So the calls should look like this for the t part:
k1=f(t(i) , etc);
k2=f(t(i)+0.5*h, etc);
k3=f(t(i)+0.5*h, etc);
k4=f(t(i)+ h, etc);
For the etc parts, I am not sure how to advise yet. Do you have two different state variables P and Q? And both have time derivatives that you know the equations for? If so, then you need to rewrite f and the k's code to account for this. Maybe separate f's and k's for the P and Q variables, or maybe carry a 2-element state vector instead.
Can you post the differential equation(s) that you are working with? Then we can advise further.
