The hard part about answer that question is what applications does he plan to run for said time period. It is easy to find a word processing/minor internet notebook with 4-6 hour battery life. On the other hand it is very difficult to find a machine that can run intensive cpu/gpu applications for 4 hours.
Currently I have a m1330 with a 9 cell battery it can easily push 6 hours with the newer 45 nm cpus. Another factor is in may-june the new chipsets will help further increase battery life even further. There is also the new ultra lower power intel cpus which might be able to do what he needs...(atom etc) The 8.9 EEE pc looks interesting when you consider they are estimate close to 8 hours when they upgrade the cpu for example. http://www.notebookreview.com/ is not a bad site to look at various reviews, combined with the forums you can often get a good gauge on estimate battery life. You also need to consider how most li ion batteries degrade over time. This means they will have to be replaced if you aim for a notebook which pushing the bare minimum. So in the end it really depends on the applications he plans to run...
