Hmmm. I'm running on the same machine (except with 8 GB RAM) and Windows 7 64-bit and it runs fine at 1280x800, Edge AA, Normal Shadows on, SSAO off, 8x Anisotropic, Depth of Field on, as well as post processing. It's very playable.
I do get the occasional stutter/lag spike after loading a new level/save game. To solve this problem I just wiggle the mouse around for two or three seconds in every direction to give it a chance to load everything into memory and it goes smoothly from there. (But running on 8GB RAM on Windows 7 also means the HD3000 has 1.7 GB of video memory available. Not sure what it is for 4 GB RAM.)
Try this: right click on the desktop and click on "Graphics Properties." This should bring up the Intel HD 3000 menu. It looks black and blue menu. I'm not booted in Boot Camp right now, if it's not "Graphics Properties" it's the other option in the right click menu.
Anyways, in here, on the left click on the 3D Tab, and set everything under here to "Performance." if there is a slider. If it's drop down, set it to off or Application Preference. There's also a slider for power management under one of the tabs to the left. Set that for max power/performance as well.
In addition, make sure you're plugged in when playing.