I've heard PR updates reported first on 2/18. There are a bunch of DCs out there and they are in no way identical, including number of pages indexed, PR of your pages, etc.
Also, if you check only your toolbar, that sends a query to toolbarqueries.google.com, which resolves to who knows what IP, and the address can change every 60 seconds. Even if there were no updates going on, query sent to different DCs can return different results.
Congrats on PR 4
