There is a big difference between Javas (required) memory allocation, and the systems require RAM load. The -Xmx:4G parameter tells Java how much memory to make available for java object creation, but does not in any way effect the memory requirements of OpenGL textures, sound assets controlled by native sound players etc.
If the -Xmx:?? allocation is too small, java will typically fail to load the modpack entirely or crash out randomly as mod pack object creation requirements peak. You can monitor this by pressing F3 and watching the allocated/used memory counts at the top right of the screen, and adjust Xmx to be larger if the used memory approaches too close to the max.
On the other hand, if your Xmx allocation is "too large" then you have committed memory to java that might, instead, have been used to cache video textures, so a too large allocation there can actually impede system performance: If you are running custom texture packs and/or shaders than having additional system RAM available for all pruposes can be helpful, but really when performance on a laptop suffers it is usually because the laptops use low performance video hardware by default...
Many laptops in fact have dual video chipsets - a low power chipset used for most software and rendering and a chipset activated for "performance" software like games - this one uses a lot more power and runs hotter and will drain the battery that much faster. Most laptop video drivers detect which kind of software is being used by looking at the process name (name of the .exe) and as minecraft is a java app, and java is usually used for enterprise software development, detect "java.exe" as something that should use the low power chipset. These laptops usually come with an extension to the video settings that lets you add games (by exe name) to a list of "high performance" titles - if you have this kind of laptop finding and setting this might have some benefit.
More usually however, Minecraft is simply limited by the performance of the graphics chipset on laptops and you just need to go and turn Minecrafts graphics settings from "Fancy" to "Fast" and turn Lighting either off, or to its lowest setting (which is still, usually, quite good). Using a texture pack is just a no-no.
