Can I use an iPhone 4 if I have an iPhone 3?

I found an iPhone 4 and we restarted it. So now it's blank. And I wanted to know if I can use it since I have the same company?

What do you mean by blank? Just connect it to the computer and iTunes should open, then after that choose the iPhone and click "restore". This will delete everything on the iPhone 4 and you have a new iPhone. That's all.