Can I safely charge an iPhone 4S with a 5V 2A source?
When I travel, I only bring my iPad charger and use it interchangeably with my iPhone 4 without noticing any battery life performance and by that extension it should be the same for iPhone 4s. 

Apple's website seems to confirm this; For the 10W USB iPad charger - "It also charges iPhone and all iPod models with a dock connector."