Source: Azzkiker from HL2Files.com via mail

Mapping for Half-Life 2 has not really begun yet, since the current SDK only really works for Counter-Strike: Source. Well Azzkiker from HL2Files.com brings us the solution. He informs me that he has put online a tutorial on how to get Hammer 4.0 working for Half-Life 2. You can check out the tutorial here at HL2Files.com.