Quite, but this is in a B lobby.
The basic math behind this is every driver who finishes ahead of you takes 80DR plus ([your DR-their DR]/500) from you. In a Daily Race C lobby finishing last on the road after a disconnect means you finish 15th, so 14 drivers ahead will take that amount from you*.
To lose 4,000DR to 14 drivers is 285 points per driver. 80 points of that is the raw amount, so you'd need to lose 205 DR to each driver - which means that ([your DR-their DR]/500) = 205, so 205 x 500 equals the difference in DR. Only problem is that's 102,500, which is bigger than any possible difference between any two drivers' DR in any lobby anywhere (100,000 - 1, or 99,999), so that's not possible.
The realistic maximum loss for that position in a B lobby is 1,680 (|80+[{30k-10k}/500]|x14)* - it's possible to lose more because it's possible for a B driver to have 100,000 points, but they would need to be B/B, not B/S. You can expect to lose 1,120 (80 x 14)*, as it's more likely you'll be behind some drivers with a higher DR (reducing the points lost) and some with a lower DR (increasing the points lost). 4,000 isn't possible.
Rational answers here include "you've previously lost more and whatever service you were using to keep an eye on it hadn't updated" - even if you're directly looking at the value on the GT website itself (which is where these services draw the data from), it sometimes doesn't actually update after each race; I've seen it update only after matchmaking for the next race. Of course even two races doesn't add up to 4k, but I suppose 3,480 (16th; 15th but last)* might look like 4k in the heat of the moment.
*I... can't recall how the game treats disconnected drivers, but you should gain points from them too, making these values too high by 80 or so points