Maximal average over surfaces of codimension 2 in $\mathbb R^4$

In this paper, we obtain sharp $L^p$ improving estimates for maximal averages over nondegenerate surfaces of codimension $2$ in $\mathbb R^4$. We also establish local smoothing type estimates for the averages, which are accomplished by making use of multilinear restriction estimates and decoupling inequalities for two dimensional conic extension of two dimensional nondegenerate surfaces.