From perusing their website, it appears that Budapest is a favourite with 4U customers...and obviously that is why they have boosted frequencies on this particular route.
How are the other routes doing? Any ideas as to which ones are the most popular?
How are the finances? According to newspaper reports, 4U should soon be getting some more cash from LH to increase its competitiveness.
Is there any chance that if 4U becomes truly successful, LH will hand over some of its low-yield Europe traffic to them? (As BMI did with bmibaby at EMA in the UK).