To be more specific, I’m trying to implement an Android application, which forces the phone to operate as an RFID tag. This tag is read by scanners placed around doors (thus the > 4 inches constraint) identifying the person and his whereabouts.
The phone will act as a smart tag, thus providing further use cases.
NFC is especially limited to the 4 inch distance. Because the Near Field Communication is seen as a “approved communication” between the two partners it is not designed to be used as a tracing device like RFID is used in stores for example.
Thus you can use NFC as a RFID device the distance would still be limited to the 4 inches by the hardware in the phones.
you could instead use the new Bluetooth 4.0 Protocol. The Setup-Time between Communication Partners is as fast as NFC (<0.1s) (in contrast to Bluetooth 2.1 with almost up to 6s)
Most of the NFC enabled Android phones have low level capacity to emulate cards. The NXP chip that is used in the phones as well as the android kernel supports card emulation. However, the high level Android API does not expose these features most likely because some obvious security problems with such feature. To be able to read existing cards and emulate them would make it possible to easily duplicate access cards and other RFID/NFC tags and even some smart cards. This is already possible with the right equipment but to put that capability in the hands of every person with an Android phone might upset whole industries. For example, all physical access control systems rely on cards that could easily be duplicated by using an android phone. I think this feature is not there because enabling it would cause HUGE upset in multiple industries.
You can see that the kernel has support for card emulation (that is what Google Wallet Tap & Pay is using to enable you to use your phone for all your credit cards etc.):
git clone https://android.googlesource.com/platform/external/libnfc-nxp
If you do low level “hacking” you can get access to these features.
To increase the reading distance of a RFID reader, you need to increase his power.
While this is technically feasible, most countries have set (by law) the maximum power of RFID readers to 1 W.
My guess would be that the NFC of your phone is already operating at 1 W and that you cannot programmaticaly force it to operate above.