MATLAB does not automatically know which values to substitute. You need to specify this in the call to subs() explicitly. Try the following code
syms x1 x2 x3 x4
g1=cos(x1)+cos(x2)+cos(x3)+cos(x4)-0.9424;
g2=cos(5*x1)+cos(5*x2)+cos(5*x3)+cos(5*x4);
g3=cos(7*x1)+cos(7*x2)+cos(7*x3)+cos(7*x4);
g4=cos(11*x1)+cos(11*x2)+cos(11*x3)+cos(11*x4);
g=[g1;g2;g3;g4];
x=[x1;x2;x3;x4];
j=jacobian(g,x);
j_inv=inv(j);
p=j_inv*-g;
X1=input('enter initial values of x1');
X2=input('enter initial values of x2');
X3=input('enter initial values of x3');
X4=input('enter initial values of x4');
disp(x)
disp(p)
subs(p, [x1 x2 x3 x4], [X1 X2 X3 X4])
