They could just say "4gb" or "250mb" and reserve the words "unlimited" until they truly are. I'm on Three and use 20-25gb/mo. They say it's unlimited, but it certainly isn't limiting.
It can never be "unlimited" because there is always a limit to how much data you can transfer in a month, given a theoretical maximum bandwidth of x Mbps for 4G networks (actual achieveable transfer speed even less). The correct term is "unmetered".
4Gb is "unlimited" fair usage. That would seem less fair expressed as "35 minutes". *
Packages offering 250MB of 4G would seem pathetic when expressed as "2 1/2 minutes of internet".
*Average 4G speed is apparently around 15Mbps in UK.