I don't yet have any publicity text for season 4, or indeed any information regarding publication dates of the region 2 discs.  Region 1 is scheduled for release on 6th January 2004.

However, for your convenience, here are the links to Amazon for all the available boxes in all avaliable formats.

Amazon.co.uk - Region 2.
Season 1, £33.74
Season 2, £41.24
Season 3, £48.39 (wow, that seems to have gone up since I bought my Christmas presents!)
The Gathering, £7.99
In the Beginning, £7.99

Amazon.de - Region 2, German-language box.
Season 1, Euro 24.99
Season 2, Euro 46.99
Season 3, Euro 39.99
The Gathering, Euro 14.99
In the Beginning, Euro 14.99

Amazon.com - Region 1.
Season 1, $84.99
Season 2, $84.99
Season 3, $84.99
Season 4, $69.99 (release date 6th January 2004)
Season 5, $69.99 (release date 13th April 2004)
The Gathering / In the Beginning (two-disc set), $19.99

Amazon.fr has each of the seasons split between two boxes, with French-language packaging, again region 2 format.  The sets seem rather expensive at around 40 euros per box (Euro 80 or thereabouts per season), and as the actual discs are undoubtedly exactly the same you'd be better buying from the German site unless French packaging is very important to you.

These prices were correct on 1st January 2004, but they do change, so check the current price before deciding on your order.  There are also some useful deals for those interested in buying more than one box at a time, in particular Amazon.com will sell you seasons 1-4 as a package for $279.94.  However, Amazon.co.uk are rather inclined to pair B5 discs with things like Star Trek, so shop around.