Can I use my Nexus 4 charger to charge an iPhone 4s? I have a Nexus 4 charger (the block with the USB port on it) lying around. Can I use it to charge my iPhone 4s? Is it going to cause any trouble?
Because USB is a standard of 5V at 100mA increments, the charge should work without any problem.  Some chargers sadly prevent themselves from working unless you use their device with their charger.  You are much more likely to find a charger decides not to work with your phone than the charger will break your phone.