# skfolio.measures.fourth_lower_partial_moment ### skfolio.measures.fourth_lower_partial_moment(returns, min_acceptable_return=None) Compute the fourth lower partial moment. The Fourth Lower Partial Moment is a measure of the heaviness of the downside tail of the returns below a minimum acceptable return. Higher Fourth Lower Partial Moment corresponds to greater extremity of downside deviations (downside fat tail). * **Parameters:** **returns** *ndarray of shape (n_observations,) or (n_observations, n_assets)* : Array of return values. **min_acceptable_return** *float, optional* : Minimum acceptable return. It is the return target to distinguish “downside” and “upside” returns. The default (`None`) is to use the returns mean. * **Returns:** **value** *float or ndarray of shape (n_assets,)* : Fourth lower partial moment. If `returns` is a 1D-array, the result is a float. If `returns` is a 2D-array, the result is a ndarray of shape (n_assets,). ### Notes NaN handling: - Unweighted: NaNs are ignored; all-NaN inputs yield NaN. - Weighted: NaNs propagate.