There was a discussion after the phone was pulled apart XDA style and we first discovered that it even HAD an LTE radio (I remember the uproar when that was found, remember it's UNLICENSED (no FCC regulatory approval)). People noticed that there was no amplifier for the LTE radio but there were solder points for it. Based on this information plus the weak LTE signals people were getting on AT&T in most areas (-107dBm or less) everyone put two and two together to get that there was no amplifier built in. The amplifier is only really needed to enhance the reception to the tower, not from the tower (you can't boost a useless signal into a usable one if the antenna can't receive the signal already, it doesn't work like that), so as long as you're in the city, you'll have LTE without a problem as you're mentioning. The amplifier is enabled and used for transmissions when you're further away from the tower and the signal cannot carry as far from the chip's existing amp (typically weak) in order to maintain a two-way transmission. This would also account for the longer battery life that people are reporting on their N4s after they switch over to LTE primary. No amp means no extra draw against the battery as the GSM radio is only used as backup and for calls on T-Mobile (they don't do VoLTE). Typically a tower will not enable broadcast in a direction that there is no serviceable clients, hence the multiple vertical aerials facing 3 directions, and will cease to broadcast at full strength until it receives contact from a client device (power savings). Either your basic amp built into the chip is hitting the tower or there's other LTE users in your area and keeping the tower at full strength, either way, AWESOME.
