Think of your rig like it was a freighter getting loaded to cross the Pacific. It is a 10,000 tonner. Since you have four cargo holds, you have approximately 2,500 tons per hold...are you following me? The ship is rated for no more than 3,000 tons per cargo hold.
Now, you can probably fit 10,000 tons of pig iron in just the #4 hold aft as it is pretty dense. Except that the ship will ride with her butt practically awash and the bow up in the air where wind and waves will push you off course 24/7, that is, unless you sink right at the dock! . OK, you decide to pull into Honolulu and add another 2,500 tons of general cargo to the #1 hold because you want to keep the bow DOWN and not have such a poor handling in wind/waves.....except you are now over the weight rating for the entire ship and when you depart and are caught in a storm and.... you sink. (this sounds to me like what you plan to do with a heavy bumper!)
Or try another scenario....your ship is empty, it will take 10,000 tons of cargo. It is rated for 3,000 in each hold, not to exceed 10,000 in total...(which is EXACTLY the idea the VIN tag on your truck is telling you!) You decide to load 6,000 tons in the aft #4 hold and another 2,000 in #3 and 1,000 in #1 & 2 holds. Hey, no problem, right? You can carry 10,000 tons and that is what you have on board. EXCEPT you have WAY too much in #3 & #4 holds and the ship sits really LOW in the stern and the bow is riding too high. Up comes that storm and again....you sink.
Now, with a rating of 3,000 tons maximum in each hold, you still can't haul more than 10,000 tons of cargo, you can just load it where it fits a bit better. Say you put 3,000 in #4, another 2,000 in #3, another 2,000 in #2 and another 3,000 in #1.....hey! you just safely loaded your ship because not only did you not exceed the 10,000 tons maximum the ship is rated for....you achieved a good BALANCE and the ship will ride close to level. Bring on the storms!
The beauty of having four cargo holds means he ship can be loaded and the cargo distributed between them to SAFELY place the load fore and aft (and port to starboard I might add!)
The bottom line is, if you have 12,000 tons of cargo, you can't just put 3,000 tons in each hold because it is RATED for 3,000 tons, you still have to pay attention to the combined weight of the cargo and the 10,000 tons rating the ship will carry SAFELY.
...or, you can get a bigger ship....and in this instance, a bigger truck.....or load only the amount of cargo the truck is rated for and compare the axle weights to see if they exceed the VIN number for each axle.
Decisions, decisions, decisions....