  We study charged black hole solutions in 4-dimensional (4D)
Einstein-Gauss-Bonnet-Maxwell theory to the linearized perturbation level. We
first compute the shear viscosity to entropy density ratio. We then demonstrate
how bulk causal structure analysis imposes a upper bound on the Gauss-Bonnet
coupling constant in the AdS space. Causality constrains the value of
Gauss-Bonnet coupling constant $\alpha_{GB}$ to be bounded by $\alpha_{GB}\leq
0$ as $D\rightarrow 4$.
