The -4% correction to MACTOW is to allow for the worst case C of G change due to fuel burn from the auxiliary fuel tank (when fitted). There are also small MAC changes for fuel burns from the other tanks but these are much less with a max change of 2%, see any manual loadsheet for details.
The maneuver altitude capability is increased by approximately 100ft for every 2% the C of G moves aft, so the technique is used to find the actual maneuver altitude.
The default C of G values (UK CAA all types 4%; FAA, -300/18.5% -400/15% -500/23% -700/22.2%.) particularly the UK CAA values are conservative and may be used safely without entering the actual MAC from the loadsheet. However if you want to find your actual altitude capability then use loadsheet MAC -4%.
I suppose it would also be correct to simply read the actual MAC from the trimwheel when in the cruise, but I have never seen this done. Not an option on the NG's as they removed the MAC scale from the stab trim indicator.
S&L