"4G" doesn't actually mean anything. "20 Mbps" means something specific. If you advertise "20 Mbps" then you should be providing twenty megabits per second. If you don't actually want to provide that then you can call it 20 Mbps burst or what have you, but one way or another you need to be providing what you're selling.
The real problem is the lack of accountability. If the first company to advertise 20 Mbps without actually providing 20 Mbps was held accountable to the offer then everything would be fine: The users who want to use 20Mbps sustained 24/7 for the whole month would be attracted to that service, the provider would have to set the price at a sustainable level or go out of business, the market works. It's the fact that we allow them to get away with offering something they aren't actually selling that creates the problem to begin with.
The real problem is the lack of accountability. If the first company to advertise 20 Mbps without actually providing 20 Mbps was held accountable to the offer then everything would be fine: The users who want to use 20Mbps sustained 24/7 for the whole month would be attracted to that service, the provider would have to set the price at a sustainable level or go out of business, the market works. It's the fact that we allow them to get away with offering something they aren't actually selling that creates the problem to begin with.