Wow, I don't know how you've been coping with the iPhone 4. I remember holding onto my iPhone 4 until I could no longer bear it (and nor could the phone lol) and ended up buying the iPhone 5S which at the time, was just released. Was perhaps the biggest and most noticeable difference that I've had from upgrading from one iPhone to another. The 6S is gonna be a good upgrade for you
If you preorder online, you may or may not get it on launch day. To get the best chance of getting it delivered on launch day, you need to hit the purchase button as soon as the pre-order service goes live on the Apple website. Best to get it done and dusted within the first 2 mins of going live as otherwise delivery times may get pushed back slightly depending on how long you leave it. I recommend setting up your payment details with your Apple ID to enable a fast checkout process.