It's kind of an awkward period as we are pretty much slap bang in the middle of the normal iPhone release cycle. The last 4 iPhones (including 5C) have all been released around September/October time. Current rumours suggest the 4.7" iPhone 6 will be released at a similar time with the larger 5.5" being released more towards Christmas. This is rumours and speculation though so take with a large pinch of salt.
Ultimately the decision on whether to buy a 5S now is down to you and whether you would regret holding on for 6 months for the 6 or if you would be happy with a 5S even when the new phone is out. Sadly we can't answer that question for you as it is a personal choice.
That said as you currently have an iPhone 4, the 5S would be a significant upgrade to you with improvements in speed, screen size, camera, siri and the touchID sensor to name a few. It would also be more future proof than your current iPhone 4 which I highly suspect will not be invited to the iOS8 party when that is released later this year
