4G of RAM under Windows XP; boot.ini's "/3G" switch

[hacksaw]

My brother-in-law bought 4GB for his old Windows XP laptop and asked me to install it for him (pair of 2GB modules - one was easy to install, the other was under the keyboard and was a pain in the neck to access ... hence him asking me to do it). After installation, on powerup I immediately checked the BIOS settings and noted that all 4GB were detected, and then booted from CD to run memtest86+ and there were no errors. Second boot was into Windows XP, and it was rock solid. No sign of trouble. Then I added the "/3G" switch to the relevant entry in boot.ini and restarted. Third boot was insane. The system was sluggish, applications would start up and hang, etc. A normal restart seemed to go fine, but on the fourth boot chkdsk fired up! Yikes. After that it was back to applications hanging and general system instability.

So I booted the thing up in safe mode and removed the "/3G" switch. Restart. Now the thing is rock solid again. What gives?