There hasn't been an official announcement yet, but there are a number of details of the Heroes IV TCG expansion floating around on the web. The official site has some small details, including the picture shown here and name of the expansion which is a combination of the two Heroes IV expansion pack names - "Storm Winds". The link to the Storm Winds page there is broken at the moment, though. There's full details of the expansion available at CCG Armory, though, which is taking pre-orders of booster packs (US$2.45 ea. + shipping). The description there contains everything you need to know about the epansion packs:

Bringing all of the excitement of the fabulous Winds of War and Gathering Storm expansions, the Storm Wind expansion boosters allow you to spice up your deck! Available in these packs are over 3 times as many artifacts than the original card sets, all of the Advanced Hero classes available in the computer games, new locations and heroes, as well as all of the creatures from both computer expansions. Recruit Dark Champions and Frenzied Gnashers, become an Assassin or a Lord Commander, and battle the ultimate guards – the Megadragons!

Each exciting booster includes 15 powerful cards to help your deck blow away the competition! Buy Legendary Advanced Heros in your town, recruit the Winds of War and Gathering Storm creatures into your army, and capture game-turning power-packed artifacts! Also included in these packs are Scrolls, Wands, and Potions – never before available in the CCG!

It looks like DGA Games has managed to extract a great amount of new content from the two expansion packs (and some extra stuff from the original games) which is good to see. There's no word on release date, yet, though obviously it can't be too far away.