I have feature data $X$ and four predictions $ (u_1, u_2, u_3, u_4) = f(X)$ with $u_1, u_2, u_3, u_4 \in \mathbb{R^+}$. $f$ is an unknown function (no assumptions on its properties) that needs to be learned to predict $u$ out of sample.
I have training data that consists of approx. 10k observations (rows) of $X$ and $f(X)$. A data-related problem is that I know that in my out-of-sample data I will observe many unseen feature values of two features $x_1$ and $x_2$ that I know to be quite important to the value of $f(X)$.
Is there a model that is well suited to learn a function $f$ and make multiple real-valued predictions $ (u_1, u_2, u_3, u_4) $, ideally such that $f$ also generalizes well to unseen data?