Because of performance reasons I want to know if it's possible to target iPhone 4 and iPad only on a iOS game.
I do know it's possible to add some flags(UIRequiredDeviceCapabilities) to applications to require Compas, Gamecenter, Gyro etc. However I believe there is no combination of these requirements that rule out all of the older devices.
Maybe requiring arch7 only via UIRequiredDeviceCapabilities is a valid option. The only downsie is that this will allow 3GS as well (which might be too slow).
For a specific game of ours it's simply not possible for us to make it compatible with the 'weaker' devices.
This is not done via the iTunesConnect interface (unless it was recently changed.) It, as you stated, done via the UIRequiredDeviceCapabilities in your info.plist (found in X-Code).
For other people looking at this question, more information is here: http://developer.apple.com/library/ios/#documentation/iPhone/Conceptual/iPhoneOSProgrammingGuide/BuildTimeConfiguration/BuildTimeConfiguration.html
(someone probably needs to do a step-by-step break down of how this is done for Unity users who don't know how to use XCode.)
You cannot restrict by device - you can only restrict by feature. I would recommend just getting your game to run on Gamecenter-supported devices, which includes the 3GS, and iPod Touch Generation 2 & 3.
If the game just runs like doo-doo on those older devices, you might want to try optimizing, or if that doesn't work, just put in big letters at the top of your iTunes description that only iPhone 4 and iPads are supported devices.
This, of course, won't stop a lot of iPhone 3GS (a phone that is still sold in stores everywhere) people from downloading and possibly leaving you a bad review.
Clarification: I edited this because I recently realized that iPod Touch Gen 2s are supported in GameCenter as well. This is a bit of a problem since the Gen 2s are pretty inferior to the Gen3 Touches.