Can I charge my iPhone 4 with iPad 2 charger?

Yes, the iPad chargers are compatible with iPhones and iPod Touches. Also the USB cables of these devices are the same.

Yes it's safe. There's active (maybe passive?) regulation circuits in all the devices that limit the amount of current. The ipad charger might charge slightly faster since the output is 5.1 volts; the iphone charger is an even 5 volts.