Check required Forge version and update it if necessary. Flans Mod 4.0 requires at least Forge 916. Download it from http://files.minecraftforge.net/. Make sure you use the latest recommended Version!
Clean up your Minecraft. Make sure only ONE version of each package is in the Flans folder. More then one will cause errors.
Delete ALL versions of Flans Mod in the Mods folder and re-download it from Flans Page (http://www.flansmod.com/). Use no other source, they may contain old versions. Make sure you use the latest Version!
Delete ALL Manus packages and re-download them from Minecraft-SMP Homepage Page (http://www.minecraft-smp.de/). Use no other source, they may contain old versions. Make sure you use the latest Version!
Check for dependency's. Most packages require a part package to run. All Flan packages require the Simple Part Package, all Manus Packages require the Minecraft-SMP Parts Package.