Sliding window algorithm problem

I am suppose to get estimates for few samples then after providing a step input at 60 and not get estimates post the window .I am enclosing my code ,please let me know what changes can be done

 clc
clear all
n=70;
M=50;
x=zeros(1,n);
y(1)=0;
y(2)=0;
y(3)=0;
phi=zeros(M,6);
theta=zeros(6,1);
z=zeros(M,1);
for i=4:n+4
    if (i>=60)
       x(i)=1;
   end 
      y(i)=1.98*y(i-1)-1.284*y(i-2)+0.272*y(i-3)+4*x(i-1)+6*x(i-2)+8*x(i-3); 
      z=[y(i);z(1:M-1)];
     si=[x(i-1) x(i-2) x(i-3) -y(i-1) -y(i-2) -y(i-3) ];
     phi=[si;phi(1:M-1,:)];
     if(i > M+3)
     theta1=phi\z;
     thetaest(:,i)=theta1;
     end 
     end

Thank You