Have you been charging the battery properly? (Charging the battery all the way up, letting it drain COMPLETELY before charging ALL THE WAY up again and not leaving the battery on the charger constantly after it is fully charged?) These Lithium batteries are very easy to mess up due to the "battery memory" phenomenon.
That being said, 4 months is pretty early for the battery to be showing these type of problems, and although rare; some batteries have defective cells that go bad. Depending on who manufactured the netbook, you should be able to get a warranty replacement from them.
My netbook takes a little over three hours to charge, but it is a 6-cell and after charging, I can get 5-9 hours out of it. Give the netbook manufacturer a call, they should get you a replacement.
Just remember- when you get the replacement- be sure to be extra careful and diligent to maintain and charge the battery properly! It will make the difference of your battery lasting 6 months, or it lasting a year or two. Laptop batteries are expensive to replace, and they sure seem to need replaced often...
Hope this helps