Your current code is hard to read and debug because you have multiple variations of this expression scattered throughout your code:
(-((V^2)/(4))*sqrt((2*g)/((y(j))^3)))
Rather than trying to track down your errors with your current code, I suggest you rewrite everything using a function handle for the derivative. E.g.,
f = @(x,y) -V^2/4*sqrt(2*g/y^3)
Then in your subsequent lines of code call this function handle as f(x(j),y(j)) etc. instead of repeating the expression over and over again. This will greatly facilitate examining your code for errors, and also it allows you to call the MATLAB supplied functions such as ode45( ) for direct comparison, as well as comparing your code to pseudo-code that can be found on Wikipedia et al.
Try to do this, then repost everything for us to look over. I can see what look like obvious errors in your calculation and usage of the k's, for instance, but rather than work with what you currently have it would be best for a rewrite first. E.g., your Euler integration line becomes simply:
y(j+1) = y(j) + h * f(x(j),y(j));
Your Improved Euler would involve the equivalent of
h * ( f(x(j),y(j)) + f(x(j+1),y(j+1)) ) / 2
on the rhs, as long as you calculate the Euler step y(j+1) first (maybe store it in a temporary variable). These types of expressions in your code will be much easier to debug for errors than what you currently have.
The RK4 code becomes much simpler and would look something like this:
k1 = f(x(j) , y(j) );
k2 = f(x(j) + h/2, y(j) + k1*h/2);
k3 = f(x(j) + h/2, y(j) + k2*h/2);
etc.
