Refer to the earlier problem.

Can you find all possible non zero real quadruplet(s) (P, Q, R, S) that satisfy the following system of simultaneous equations?

2*Q = P + 19/P, and:

2*R = Q + 19/Q, and:

2*S = R + 19/R, and:

2*P = S + 19/S.

Bonus Question:

In the problem given above, if the number 19 was replaced throughout by a real constant M > 1, what would have been the possible non zero real quadruplet(s) (P, Q, R, S) that satisfy the given set of equations, in terms of M?