Something like this?
(* MinMax for x, y, z, p *)
MinMax /@ Transpose@DataxyzStein
(* {{-0.495, 0.495}, {-0.495, 0.495}, {-0.305, 0.305}, {0.299106, 0.998006}} *)
(* Select positive x values *)
DataxzSteinPosX = Select[DataxzStein, #[[1]] >= 0 &]
MinMax /@ Transpose@DataxzSteinPosX
(* {{0.005, 0.495}, {-0.305, 0.305}, {0.302262, 0.998001}} *)