Watts can be misleading in a headphone amp, as headphone drivers are often driven by voltage rather than current. So, you're likely to see output measured in voltage rather than watts.
In terms of what it can drive, IMO the HP4 strains with the AKG K-1000. Then again, that headphone wants a 7-10 wpc speaker amp. Aside from that, the HP-4 will drive any dynamic headphone.
My personal favorite headphones with this amp include the Sony R10, Grado HP-1, Grado RS-1, Sony CD-3000, and Audio-Technica W2002 (finally! An amp that makes this headphone sound good to me)