Personally, if you can't wait I still recommend the 5s or the 5c because they are still pretty good phones & offer a much more immense performance compared to your 4s. Both the 5c and 5s have a 1.3 Ghz dual-core processor whereas the 4s only has a 1.0 Ghz dual-core. They also have a larger 4 inch display (your 4s has a 3.5 inch display). Their perfomance is top-notch, battery life is surprisingly excellent (much better than the 4s) and overall the ios 7 software on them works wonders, whereas the 4s will let you upgrade to ios 7 but because it can't handle its power properly, its battery life is compromised unlike the 5c and the 5s which were already built to run ios 7. So are you sure you'll be patient enough to wait for the 6?.. Besides, you'll find the 5s or atleast the 5c cheaper to upgrade to once the 6 comes out making it even more worth buying them? I have an iphone 5c and its the best phone I've ever had. I've had 3 android phones, 2 windows phones & 3 blackberrys (all of them were great all-rounders btw) but my 5c is by far the the most seemless phone i've had in every way.
I also love the available colours (mine's green) & it's definitely a success since so many of their units are sold. You might have seen the 5c being advertised absolutely everywhere (phones shops, billboards, and still on telly even) even though its been well over 6 months since its release. People who don't have the 5c think it's cheap-looking but when you feel it yourself you'll realize that it feels very robust and underneath that polycarbonate plastic (quality plastic), is a hidden steel structure which pretty much makes it a premium-built iphone anyway. Besides, the 5c is the improved iphone 5 which itself was premium-built from aluminium but turns out that the anodized colour chipped off which is why apple came up with the idea of producing the iphone 5c the 1st place, not only for the intention of building a budget iphone because the 5c is still pretty expensive anyway. That should explain why it's a quality handset.
However, if the 5c isn't your cup of tea, definitely consider the 5s. The 5s basically has a more resilient anodized colour over its aluminum than its predecessor, the 5 did. However, the 5s is the best because unlike the 5 and the 5c, it has a 64-bit chip, improving its graphics for multimedia, especially for gaming. It also has a dual-tone flash (cold & amber), resulting in the images taken having the right colour temperature. The 5s's homebutton is the most obvious change compared to all previous iphones however. The home button is fingerprint-scanner made from saphire with a gold ring around it. Besides, I think the 5s will receive the upcoming update the iphone 6 will have anyway according to most official sources, so if i didn't manage to convince you with the 5c, it's still definitely worth considering the 5s.
Pheww!..now let's speak of the iphone 6. The display is definitely going to be bigger but apart from that apple hasn't giving out much. However, would you like a display bigger than your 3.5 inch, for a matter of fact, even bigger than 5c and 5s's 4.0 inch display?..definitely have a think about that since the 6 will have a larger display. I think my 5c's 4.0 inch display is just about the right size for me to operate my fingers around. I usually prefer a touchscreens which are nimble to operate and carry in your pocket. I think that was one major up-point with your good old 4s.
However, if you can wait then there's absolutely no harm in going for the 6. Just rem The only barrier is your patience. Hope this helped.