Does configuring the architecture to ARMV7 restricts the app to be installed only to devices starting from 3Gs/iPhone 4? I'd also like to know if there's another way to enable device version restrictions? Thanks for the help!
If you restrict the application to only support ARMv7 devices via the armv7 entry in UIRequiredDeviceCapabilities, that won't stop support at the iPhone 4 level, it will also support the iPhone 3G S and 3rd Generation iPod touch. If you really want to restrict to just the iPhone 4 and newer, you could use the front-facing-camera hardware filter, although that would also drop out the original iPad.
A full list of devices and what options they support can be found in the iTunes Connect Developer Guide, Appendix C: Device Compatibility Matrix. From that matrix, you can see which devices will be blocked for which combination of required hardware capabilities you specify.