I trust you meant "4gigs ram."
One thing to check is that the fans are working and the vents are clear. When you are using it, make sure it's on a surface where none of the vents are blocked. Overheating is the most common cause of unexpected shutdowns. Perhaps it's implicated in this case, also.
It will likely be impossible to determine the cause of one random computer lockup. I would recommend using a light-weight system monitor such as Gkrellm or Conky, turning on the temperature monitor, and keeping a wary eye on the indicators. (The temp monitors won't work unless the computer has temp sensors, but most contemporary laptops do.)
Last edited by frankbell; 01-23-2012 at 09:45 PM.