There is some great investing advice out there, and of course, some of it is pretty bad. In the latter category, you might have, "It just doesn't matter how high the price is -- buy all the Enron you can."

And while you can spend all day listing smart and useful investment advice, I got to thinking about great advice that is limited to only four words.

Let's check the results.

"Buy what you know"
This is probably the second-most-famous four-word piece of investing advice. This advice comes from, or is at least most popularly attributed to, Peter Lynch's One Up on Wall Street. As Jeff Fischer describes at great length in a timeless article written several years ago about this phrase:

"[I]t is most often read to mean buy the brands that you know, buy the companies that make products that you like, and buy the company names that you always hear in daily life.

When large-cap stocks are soaring, this strategy, simple as it is, appears brilliant. 'If I just buy Coca-Cola (NYSE:KO), General Electric (NYSE:GE), and Hershey (NYSE:HSY), I could double my money every three years!' Of course, when large caps go into long periods of rest or retraction, the strategy requires patience and offers less than blistering returns, especially if you 'bought what you knew' as it was hitting a seven-year peak.

Buy what you know is one-dimensional advice for three reasons. First, what you know may not be worth investing in. Second, the practice of buying what you know is rarely interpreted to mean buy the business model, the cash flow statement, and the balance sheet that you know backwards and forwards. It too often is seen as 'buy your favorite brand.' Period. If you happen to know and love Kmart, but you didn't learn about its financials, you [were] in a sorry situation because you were an uninformed investor. Third, I've never heard the term 'buy what you know' coupled with anything regarding valuation. It seems to be 'buy what you know -- at any price.'"

Thank you, Jeff. Buy what you know may help new investors get comfortable with the process, but it simply won't help you pick particularly good stocks without getting into the valuation side of the equation. Plenty of people bought KrispyKreme because they knew it. And plenty of people bought Starbucks (NASDAQ:SBUX) because they knew it. Acting on "buy what you know" doesn't lead you anywhere special.

"Buy low, sell high"
I'm pretty sure this is the most famous four-word piece of investing advice ever. Who knows who first uttered this quote? (I don't, and I researched it for a solid five or 10 minutes. Email me if you know the answer.)

This advice is both perfect and largely useless. By definition, if you succeed in buying low and selling high, you've made a profit. Any purchase is made with the expectation -- or at least hope -- that in absolute dollar terms, you're going to be selling at a higher price than what you've bought for. But since the advice itself gives no guidance to what is "low" and what is "high," it can't be used without a whole lot of addendums. Buy stocks with low P/Es, or at 52-week lows, or during bear markets, or any number of other interpretations of "buying low." Selling high might or might not be useful advice. After all, as Philip Fisher has famously written and as adopted by Warren Buffett, the best time to sell a stock, if it's properly researched, may be almost never.

We can all tell plenty of stories about someone selling a stock at a quick profit that seemed high, but turned out to be several hundred or thousands of percent below what could have been made by holding onto the stock. Tom Gardner frequently mentions Dell (NASDAQ:DELL), Websense (NASDAQ:WBSN), and Sanderson Farms (NASDAQ:SAFM) when confessing his own bad calls. Not to pick on Tom -- his results speak for themselves. But these were mistakes that came out of the "buy low, sell high" mold.

"Buy an index fund"
This is the most actionable, most mathematically supported, short-form investment advice ever. If you look up The Motley Fool in the encyclopedia -- or at least Wikipedia -- you'll find that we are "famous for [our] view that, for the majority of people who have little time to keep track of stocks, the best investment strategy can be summed up in four words; 'Buy an index fund.'"

And that remains true. If you've got little time to keep track of stocks, this really is the best investment advice around. It's not perfect -- after all, you might be asking, "Which index fund?" And then you'd want to specify certain characteristics, such as:

  • No-load
  • Low annual cost
  • Low turnover
  • Broad index

That means a fund like Vanguard Total Market Index Fund, which coincidentally may hold a lot of what you know, including GE and Coca-Cola.

When cornered at cocktail parties for investment advice, this is the one I usually provide. After all, barely 25% of mutual funds over time beat the relevant market index. I don't think that you can really improve on this advice if you're stuck using four words or fewer.

But you can spend more than four words on investment advice, and as with the other four-word mantras above, if you do so, you usually get even better advice. Like the classic index fund, a managed fund can be no-load, low annual cost, low turnover, and well diversified. It can, on rare occasion, be managed by someone or some team that has the ability to properly allocate capital and value businesses and thereby add value over and above what the market average provides on its own. When you combine all of these factors, you get that rarest of finds, a mutual fund that improves on the index fund and becomes something that will help you make money.

And they are out there. They take a bit of work to find (more than can be summed up in four words), but Champion Funds actually uncovers those incredibly elusive market-beating funds. So far, more than three quarters of the mutual funds Fool fund guru Shannon Zimmerman has recommended are beating their benchmarks, earning 20% returns compared to 10% for the yardsticks over the same time period.

You can do just fine by sticking to an index fund, but you can do even better with a managed fund that adheres to the same principles that make the index fund so hard to beat, but also has exceptional management. Click here to learn more.

Bill Barker does not own shares in any of the companies mentioned in this article. Coca-Cola is an Inside Value recommendation, Dell is a Stock Advisor pick. The Motley Fool has a disclosure policy.