In order to continue your multiplayer save with more than 4 players, create a new lobby, set your party size and start a new game. Then load your main save, this will carry over the player limit. Ingame sessions unfortunately don't allow you to tweak the settings.

Sildur's Shaders is an extension of the GLSL shader mod for Minecraft, now part of Optifine. Vibrant shaders completly revamps the lighting system of minecraft and adds advanced effects like volumetric lighting, bloom, ambient occlusion and reflections. While still maintaining high performance.