I read recently that Exchange databases, like the Public and Private Information Stores, are highly susceptible to corruption when exceeding 4GB. Our Exchange 5.5 (Standard Edition) claims a 16GB limit for Information Stores. Should I start to worry as it approaches the 4GB watermark?
I don't think there's really any barrier at 4GB. I've seen plenty of Exchange systems running in excess of 4GB without any difficulty whatsoever.
The one barrier I have seen is when a .PST file approaches (and reaches) 2GB you will see a lot of problems.
So...I wouldn't be concerned about a 4GB milestone, as long as you have plenty of free disk space. But apply the service packs and have good backups anyhow. :)