4 Gt is roughly all the anthropogenic CH4, responsible for about a third (or possibly a half) of temperature rise. After that we will continue at a lower level in order to keep curating the level and as a precaution against methane bursts.
4 Gt of CH4 is considered roughly equivalent to 120 Gt of CO2
Fascinating - saving the planet by adding iron chloride to the mix of pollutants emitted by diesel-powered tanker ships in order to convert methane into CO2. I can't imagine many more ways in which the approach could be unintuitive, but if it works and the side effects are thoroughly understood, I appreciate the pragmatism!