What are the pros and cons of SP4 on SMS 2.0?
SMS 2.0 SP4 is a cumulative patch, incorporating all of the hotfixes released post SP3. So, if you haven't applied the numerous hotfixes since applying SP3, then SP4 will make your SMS environment more stable.
There are really no cons to applying SP4, only caveats. You just need to prepare for the installation. Read through these stories from folks who have already done it and given their insight.
And, read about those post-SP3 hotfixes that Microsoft did not include in SP4.