I've got a couple of insurance agents in the family, and I've asked this question. Are they going to break out a scale? No. But if the adjuster can demonstrate that you're over the limit, you are screwed. It's not hard to type in a boat model, or camper into a computer and come up with a weight. Geico adjusters in particular are a-holes, and they do a lot of internet "research" to lowball you as much as possible. Like a buddy with a restored '64 Impala that was offered 4k to total it out (based on Biscayne prices, as far as we could tell), or another with a $3k mountain bike that was offered a price based on Walmart's selection (like $150 or something). If they can get out of paying, they'll try anything they can...that's their job. If you feel like rolling the dice that you'll arrive safely, or that your adjuster is a moron, more power to you.
Safety-wise, there's probably not an issue, if you can drive with 3500 lbs you can probably handle 4000 safely. It comes down to steering and braking more than drivetrain. But yeah, if it was more than a mile or two, I probably wouldn't. There are WAY too many stupid people on the roads.
'12 Sport...I need to up my powerball investments, mods are expensive!