In my experience this is false and also depends on the market and your specialty. (Sorry, I don't have data - but it is my experience when looking at freelance roles in the US or when talking to my North American counterparts.)
For example in the United States, employees in many states have much less security than those in countries like Australia. My experience is that freelancers can charge 1.2x to 1.5x the hourly rate of their full-time employee counterparts.
In places like Australia, where employees have a lot of security and there's no such thing as "at will" employment, freelancers are able to charge 2x minimum the hourly rate compared to their full-time employee counterparts. This is because employees get a lot of benefits, like 4 weeks of leave, premium health insurance, sick leave, up to 6 weeks notice for dismissal and redundancy payouts etc.
I'm in a high-demand, high-value technical role and able to go above those multipliers here in Australia - but on average, say for a middle-tier generalist Java developer, 2x is very common.
The only way I find you're able to charge 4x or more your hourly rate, is if you do public speaking, write books, author a blog and participate as a (dear I say it), "thought-leader" within your local tech community. At that point your title becomes consultant and you generally only cater to clients who are hopelessly clueless in your area and want the best advice, or those with a large bank-roll to "get shit done, properly, fast".