function p_s = region4_pcalc(T)
% Description:      Calculates the saturation pressure at a given
%                   temperature based upon the IAPWS IF-97 Release for 
%                   Region 4                
% -------------------------------------------------------------------------
% Syntax:           p_s = region4_pcalc(T)
%                   T           Temperature [K] 
% -------------------------------------------------------------------------
% Output:           p_s         Saturation pressure at T [MPa]           

n = region4_sat;

upsilon = T + n(9) / (T - n(10));

A = upsilon^2 + n(1) * upsilon + n(2);
B = n(3) * upsilon^2 + n(4) * upsilon + n(5);
C = n(6) * upsilon^2 + n(7) * upsilon + n(8);

p_s = (2 * C /(-B + (B^2 - 4 * A * C)^(1/2)))^4;