According to the official wiki,

• Scotsman's Skullcutter
• Claidheamh Mòr
• Half-Zatoichi
• Eyelander

From the Obtaining Demoman Achievements page on the official TF2 wiki:

Left 4 Heads

Decapitate 4 players with only 10 seconds between each kill.

Difficulty: Hard
How to obtain: Equip the Eyelander, Scotsman's Skullcutter, Horseless Headless Horsemann's Headtaker, Claidheamh Mòr, or Half-Zatoichi and engage multiple enemies at close range. Pair your melee weapon with the Chargin’ Targe in order to reach new targets before the ten seconds expire.