Anything can get hacked, especially with crimes like this on the rise in the crypto world. But honestly, $4 million might not be that huge for them- it could just be part of their daily transactions since withdrawals are automated at this casino.
A good casino can always cover its losses, from its own money or with a quick loan. I don't know if $4 million is a little or a lot for this casino, in any case, it's good that they covered it and showed that they are serious about what they do.
I agree with you that anyone can be hacked, even easier if they have an insider. With hacks this big, I always suspect that someone from the inside helped... I doubt we will ever find out, we never get the whole story in the end because of security reasons. In any case, I hope they manage to get the money back from the hacker... sometimes it is possible if the hacker makes a mistake.