I'm borrowing a bass from a friend of mine for 2 and a half weeks.
Now i've been playing for like 4 hours a day and i'm a beginner. My
fingers hurt a little(this is the 2nd day i've had it at home).
Is it unhealthy for the hands/fingers to play for so long a day?

*and please forgive my bad english

apparently regular playing keeps arthritis at bay, so playing must be good for your fingers. you just need to get past the blisters and aches, then your fingers will get used to it.

Thump that shit with your thumb and call back in a week!

It all depends on your technique. If you do the wrong movements, you could get problems.

4 hours is a lot of time. The longer you play: the less concentration, the more tired, eventually wrong movements. Just be careful!

If you're a beginner, you're not really in danger of any long-term damage, you're just using parts that you don't usually use. Be sure to do some stretches and a couple of coldies and you'll be good.