During my undergrad we had a physics professor who would only use si units. 3,000 km became 3 megameters. A billion dollars were a gigadollar. To this day I don't know how I feel about it.
I guess, but that's longer than “4,000kg” and unusual to boot. Anyone reading it is going to convert it in their head to 4,000kg anyway. Four million grams is not something relatable. Hard to picture the weight of millions slips of paper vs. a few thousand large water bottles. Or roughly two sedans.
The megagram is on the same list as the dekaliter and the hectowatt. They don't exist :)
