How long does it take for 4 bunnies to wear a concrete flooring? I'd love to make a new pen with a concrete floor but I'm afraid my bunnies will break through to the earth beneath
Rabbits cannot break through concrete floor. That being said, concrete isn't a very pleasant flooring to use. For some recommended parameters on enclosure set-ups, see here: Housing - WabbitWiki
Concrete absorbs wetness... that means urine can soak into concrete. Just something to bear in mind.
Running around on concrete can be good for wearing down a rabbit's toenails.
I assume you're planning on housing outdoors? You may want to see samples of outdoor housing setups here on RO: https://www.rabbitsonline.net/threads/outdoor-cages-post-your-photo.96732/
As @John Wick mentioned, bonding is necessary for rabbits to share a space. One can't just put 4 rabbits together and expect them to get along. They will only appear to get along when they are babies and before their hormones kick in (as early as 10 weeks of age). After hormones kick in, they need to be separated until they are all fixed otherwise they may fight viciously (can even kill each other). Then about 2 months after fixing them, they can be re-introduced to each other using a bonding process. Pairs are considered ideal. Groups can be very difficult and success depends on having unusually submissive personalities.
I have my rabbits on concrete flooring and I tape cardboard down with paper tape over plastic sheeting to waterproof it. The cardboard gives them a bit of cushion to walk on. Also a layer of hay or straw is helpful too. They do not mind the concrete at all, but mine is not sealed. In our next set up, I intend to use concrete again, but I'll have it sealed and waterproofed for easy of cleanup. I have a rabbit that digs a ton, so have a breakout-proof floor is priority for me.