find a niche skill that charges $200/hour, there is your $400k.
takes years of experience to work up to that kind of billing rate though, and you need a little luck to be in the right field, and have chance to work for the right client..
$200/hour is pretty standard rate for a Big5 firm (IBM, Deloitte, Accenture etc) charging a client for a senior resource, with say 10 years experience, with a skill set that is not 'commoditized', such as database admin, accounting etc (too many people can do it).... but freelance rates (like LZ's situation) are usually lower.
You need to have skill in some new technology, and $200-300/hour is still within reach.... or, you can travel to work in Saudi or Dubai where they pay you with petro dollars, which is usually higher than the rates in US and Europe.
That's why I have been saying, for most people here who are highly educated professionals, the low hanging fruit is definitely in the profession that you are already doing.... if you are able to increase the hourly rate from $50 (your $100k salary job), to $100, $150 or $200 - it takes a few blocks of rental houses to equal that increase in cash flow..... and you don't have to lift the hammer to knock down 1 nail... usually you just do the same thing as you are already doing in your $100k job.
