I think your 4% number is off a little bit, but no doubt water vapour is responsible for the greatest amount (about 70%) of retained thermal energy.  CO2 is about 25% and CH4 is about 5%.  Unfortunately there is nothing we can co about water vapour, which by the way is not increasing by any significant amount as are CO2 and CH4.  However we CAN do something about CO2 and CH4.  I would also note that each GHG only absorbs energy at certain frequencies.  CO2 absorbs it near the peak of the earth's radiative spectrum.  CH4 absorbs it at wavelengths where there is little radiative energy and this is why it is responsible for only 5% of the GHG effect.