deltaTsub = -2;
Tc = 310;(*in K*)
NSolve[{
      T[4] == Tc + deltaTsub,
      p[4] == Evaluate[PTL[Tc, 0.5] ] ,
      h[4] == Evaluate[HTP[T[4], p[4]]],
      s[4] == Evaluate[STP[T[4], p[4]]]}]

HTP = Interpolation[Flatten[Table[{{T, p}, Enthalpy1[T, p]}, {T, 200., 330.,1.}, {p, 0.01, 0.65, 0.005}], 1]];

NSolve[{T[4] == 308, p[4] == 0.488584,
  h[4] == InterpolatingFunction[{{200., 330.}, {0.01, 0.65}}, <>][
    T[4], p[4]],
  s[4] == InterpolatingFunction[{{200., 330.}, {0.01, 0.65}}, <>][
    T[4], p[4]]}]