Wow, Warlord, you're wasting your time thinking about this.
Well, it is a nice suggestion, if it gives 4mill it is fine. 8 sounds too much cause for newer players it would be too easy to go to the xp cap instantly, doing only 700k each gen. Could do the cap easily in 2 weeks then.
4mill actually sounds nice and I would do that with alt characters as well. Btw I would suggest that you cannot go higher than level 32 by that system because we would otherwise have too many high lelvers if it is only about money and not time anymore (though money also depends on time).