Methane is a 30x stronger greenhouse gas than CO2. So if 4% of the converted methane leaked into the atmosphere, you would be worse off (from a climate heating perspective) than if you had done nothing.
By CO2 equivalence in the context of greenhouse gas potential [1]. And, the 30X factor is only valid if you look over scales of atmospheric persistence of 100yrs. If you look at scales of 10years (the amount of time methane persists, the GHG potential strength a over 80X.
Liquid (Cryogenic) natural gas tankers and storage emit "boiloff" gas. Some of this can be burned in the 'dual fuel' propulsion engines when combined with a small amount of diesel "pilot" fuel, but not all and I'm uncertain at what quantity. Even engines that burn NG gave methane "leak by" that escapes into the atmosphere. It's not great, and no, nobody is enforcing containment via satellites at the LNG shipping level (despite a comment to the company trary above).
Hydrogen production is a much better option for dense energy storage option.
https://www.frontiersin.org/journals/plant-science/articles/...