Doesn't really make sense to me either, but who knows where that rumour came from?
Then again, the Python is maybe half the size of an Anaconda, but only 12.5% lighter, so maybe the Scarab is made of "Python stuff". I think the game does take some.. liberties.
It would be quite limiting if it does weigh 4T, it'd mean if you're flying a Sidewinder you'll need to use 2 of your 3 internal compartments for cargo racks if you want to bring back any mined materials etc.
On the Twitch, 1 hr 23 mins 48 seconds approx http://www.twitch.tv/elitedangerous/v/9892949