Interesting, my laptop doesn't crash when running the modpack (well, it does, but only when I open a gui that I haven't opened in a while, usually once every 2-3 hours), and I have 4 gb of ram, 3gb heap. Try deleting all the voxel/litemod mods. They might be causing memory leaks, which slowly build up ram usage until it has your entire heap "in use" when in reality it's just stuck, unable to be collected by garbage collection, at which point java crashes.
And that crash report is about voxelmap, I would recommend deleting voxel everything, and then installing REI's minimap if you still want a minimap.
Edit: Also note that the normal rules of "better fps with less heap" don't apply with modded minecraft, because mods add memory leaks, which means that you get out of heap errors far sooner and far more often. Mystcraft is a big one, particularly when traveling between dimensions due to loading/unloading. That's probably one of the reasons why one might crash on exiting a dimension.