Depends. The 5S may have some cool features we just don't know yet, but it certainly will at least come with iOS 7 which may be enough of a spring coat of paint to fufill the 'bit bored of this' feeling.
If your not so much tied to Apple's eco-system (i.e. spent a lot of money on apps), and are willing to try something new - then by all means go for the S4.
But as much of the smartphone experience is actually in the third party apps we buy and utilise as in the hardware and specifications, so if there are applications you utilise daily that were expensive and you don't want to purchase them again, or are apple/iOS applications and you need access to them (iMessage perhaps) then just wait till the iPhone 5S is released and see how you feel then.
Of course you could consider this. If your 4S is in good condition, and you don't feel like having another fixed contract - you could always sell it when the iPhone 5C comes out and put the cash towards buying the 5C outright (if it does come out contract free for $399 - you could end up paying $200 cash and sell your 4S for $200 etc..) making the price about the same as you would be paying for a subsidised contract phone. You can then play with a new iPhone, a new operating system and see how you like that. If it turns out its not for you - return it or sell it on for a minor dip in price, and put the cash towards a contract phone then......
Also if there are apps you need, check out the google play store and see if they are available on the android platform - and / or if there is a cost involved re-purchasing these - factor it in to the overall cost and see if you feel the move to android is worth that to you.