I also doubt that forcing you to use 4G will bring you over your limit is a revenue tactic. It's like the old trick question: What weighs more? A pound of lead or a pound of feathers?
They, of course, weigh the same, as it's said that they both weigh 1 pound. People will infer that the lead weighs more than the feathers, because lead is dense and feathers are not. Same goes here, it's all perception. 2 GB is 2 GB, no matter how fast it is transferred to you. It can also be argued that 4G uses less data, because there will be less time-outs and refreshes required to get the same data.