I tried to save the first episodes subtitles of La prima squadra. I know for a fact that the lesson is almost under the limit of 4000 words. That’s because I use the Word to get rid of nonsense number sequences and characters like : , >.
Anyway as I imported the lesson it spilt it in 3 parts.
As far as I’m aware the limit is 2000 words and has been for quite some time, I don’t remember it ever being 4000 in the time that I’ve been using LingQ (1-2 years or so).
Actually I have been on lingq for 5 years and counting. When I started it was 10 000 words however that caused some problems with the functionality of the site so it was radically reduced to 2 000. That caused a small backlash and so at one point (can’t remember when) it was increased to 4 000.
Yeah, I knew it was higher than 2000 at one point and was reduced due to functionality issues, I’ve heard Steve discuss it a bit in some of his videos and live broadcasts. But I didn’t know it had been raised to 4000, interesting. But, I’m pretty sure it’s back to 2000, as far as I’m aware.
It is 4000 at least according to the warning you get when you try to import text that is over 4000 words. I just imported a text that is 3100 words just fine.
What I have noticed and OP maybe this is what you are seeing too, is that even though the limit for 1 lesson is 4000, if you try to import a lesson that is over 4000, those lessons get split to no more than 2000 max. For example, I imported a lesson with around 5000 words. One would think it would split to 2 parts, one with 4000 words and a second with a 1000. Instead I got 3 parts of 2000, 2000, and 1000. So maybe the splitting algorithm has not been updated 
Interesting, thanks for the info, I’ll have to try importing a longer lesson sometime, I’ve always manually split my lessons because I don’t like how the auto splitting works so that’s probably why I’ve never imported anything longer than 2000 words.