Trying to extract the first four roots of an transcendental equation, but could not able to decide which are the first four roots

Question

I am trying to find the roots of the transcendental equation, for that, I have used NSolve function, but when I search for roots of b between 0 to 20, it will produce empty list, If I change the search range to 0 to 10 instead of 0 to 20, I am getting some roots. I am interested in finding the first four roots of the transcendental equation P. How to achieve this? I have plotted my transcendental functions it seems there are so many roots, now I don't know which is my first root. I am interested in extracting first four roots.

  P = -4 b^7 (2 K1 Cosh[b L] Sin[b L] Sinh[b z1]^2 + 
         Sinh[b L] (-K1 Cos[b L] + K1 Cos[b (L - 2 z1)] + 4 b^3 Sin[b L] -
             K1 Sin[b L] Sinh[2 b z1]));
    K1 = 1*^12;
    z1 = 1/3;
    L = 1;
    Plot[P, {b, 0, 20}]
    NSolve[P == 0 && 0 < b < 20]