critical lattice models are believed to converge to a free field in the scaling limit, at or above their critical dimension. this has been (partially) established for ising and $phi^4$ models for $d \geq 4$. we describe a simple spin model from uniform spanning forests in $\mathbb{z}^d$ whose critical dimension is 4 and prove that the scaling limit is the bi-laplacian gaussian field for $d\ge 4$. at dimension 4, there is a logarithmic correction for the spin-spin correlation and the bi-laplacian gaussian field is a log correlated field. the proof also improves the known mean field picture of lerw in d=4, by showing that the renormalized escape probability (and arm events) of 4d lerw converge to some "continuum escaping probability". based on joint works with greg lawler and xin sun.