def _Backward4_T_hs(h, s):
    """Backward equation for region 4, T=f(h,s)

    Parameters
    ----------
    h : float
        Specific enthalpy, [kJ/kg]
    s : float
        Specific entropy, [kJ/kgK]

    Returns
    -------
    T : float
        Temperature, [K]

    References
    ----------
    IAPWS, Revised Supplementary Release on Backward Equations p(h,s) for
    Region 3, Equations as a Function of h and s for the Region Boundaries, and
    an Equation Tsat(h,s) for Region 4 of the IAPWS Industrial Formulation 1997
    for the Thermodynamic Properties of Water and Steam,
    http://www.iapws.org/relguide/Supp-phs3-2014.pdf. Eq 9

    Examples
    --------
    >>> _Backward4_T_hs(1800,5.3)
    346.8475498
    >>> _Backward4_T_hs(2400,6.0)
    425.1373305
    >>> _Backward4_T_hs(2500,5.5)
    522.5579013
    """
    I = [0, 0, 0, 1, 1, 1, 1, 2, 2, 2, 3, 3, 3, 3, 4, 4, 5, 5, 5, 5, 6, 6, 
        6, 8, 10, 10, 12, 14, 14, 16, 16, 18, 18, 18, 20, 28]
    J = [0, 3, 12, 0, 1, 2, 5, 0, 5, 8, 0, 2, 3, 4, 0, 1, 1, 2, 4, 16, 6, 8,
        22, 1, 20, 36, 24, 1, 28, 12, 32, 14, 22, 36, 24, 36]
    n = [0.179882673606601, -0.267507455199603, 1.162767226126, 
        0.147545428713616, -0.512871635973248, 0.421333567697984, 
        0.56374952218987, 0.429274443819153, -3.3570455214214, 
        10.8890916499278, -0.248483390456012, 0.30415322190639, -
        0.494819763939905, 1.07551674933261, 0.0733888415457688, 
        0.0140170545411085, -0.106110975998808, 0.0168324361811875, 
        1.25028363714877, 1013.16840309509, -1.51791558000712, 
        52.4277865990866, 23049.5545563912, 0.0249459806365456, 
        2107964.67412137, 366836848.613065, -144814105.365163, -
        0.0017927637300359, 4899556021.00459, 471.262212070518, -
        82929439019.8652, -1715.45662263191, 3557776.82973575, 
        586062760258.436, -12988763.5078195, 31724744937.1057]
    nu = h / 2800
    sigma = s / 9.2
    suma = 0
    for i, j, ni in zip(I, J, n):
        suma += ni * (nu - 0.119) ** i * (sigma - 1.07) ** j
    return 550 * suma
