Service packs are cumulative. You can upgrade straight to SP4.

That being said, even though they have a license for SQL 2000 they will not get support. Mainstream support ended in 2008 and extended support ends next summer. They would have had to bought into extended support to get it (before that cycle started) and there is still no guarantee that a patch for an issue can be provided for a particular issue.

I would highly recommend breaching the topic of upgrades to a supported version.