The 3500 has a smooth rounded shell and no wavy bumps under, just a bit lighter and hobbypartz has it on sale now for $18. I only bought it because the 4000 was out of stock and I was impatient.
I'd get the 4000 again because it's a small difference in price and additional run time. Also the wavy bumps fit the chassis holes on the TA05 and TB03 battery side if you have those RCs.
Had to solder dean's/t plugs to both of them though, they came with the bullet plug type.
There's also a hobbypartz coupon code for 5% off, "hobbymodel".