A lower altitude will provide a wider margin between low-speed stall buffet and high-speed mach buffet, and a higher load factor available before stall. At optimum altitude the load factor at stall buffet is only 1.3 or 1.4 Gs (depending on the operator; we use 1.3).
At 4000' below optimum altitude, maneuvering G available increases from 1.3 to around 1.6 (varies somewhat with weight), according to our performance charts.