May be catching this thread a little late, but my Fijitsu Siemes Amilo Li 1818 2G also has terrible battery life. Might just scrape 2.5 hrs. Just a fact of life with these cheap laptops I'm afraid. Think the battery in mine is either 3 or 4 cell, and when you think some laptops have 9 cell batteries.
P4s are terrible for power consumption btw (mine is a core duo, which explains why mine lasts a bit longer). All you can do is turn the screen brightness down either further and if the laptop has a power saving mode (mine has a button which limits cpu/gpu speed) then make sure it's turned on as soon as you go on battery. And set the screen to power off after 1-3 mins on battery. Also disable all unnecessary background tasks. Definitely buy a better battery if there's one available.