Quiagen uses a modified cohesive end unit measurement to define the units of their T4 ligases. From my previous correspondence with them: "One Enzimatics T4 DNA Ligase cohesive end unit is equivalent to approximately 3 cohesive end units as measured with a Lambda-Hind III DNA fragment substrate in 1X T4 DNA Ligase reaction buffer. One Weiss Unit is approximately equivalent to 22 Enzimatics cohesive end units." (Enzimatics is the subsidiary of Quiagen that produces their T4 ligases). I hope this helps.

Thanks!