Minecraft, my RAM, modpacks, and FTB?

i use an HP Touchsmart 320 with 4
Gigs of ram. (Im not sure what processor and whatnot) it was Windows 7. I tried to play FTB Infinity (161 mods) and it lagged badly on the home screen (single player, multiplayer screen) i managed to create a world and it took more than 10 mins to create a world and never loaded so i just closed minecraft. What should i do? I also used the Curse Launcher/Voice beta thing to launch the pack if that helps.

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