I thought it might be instructive to post part of my reply to him here so that newcomers to the debate have a better idea of what we're talking about.
Sorry we occasionally fall back to jargon. Slice and Dice (or 4X25 allocation) is essentially an equal weighting of large growth, large value, small growth and small value stocks in your portfolio based on CRSP deciles (which break down the market into 10 slices containing equal numbers of companies) - these slices are broken down based on the market capitalization or dollar value of each company. Likewise to determine whether they are "growth" or "value," companies are divided according to their risk or volatility levels.
Total Stock Market funds (TSM) are just what they sound like, index funds that represent by market capitalization every stock in the market (best represented by the Wilshire 5000 index, which is actually ~ the "Wilshire 7000"). In other words, with TSM, every dollar the market owns across the market, your fund owns in the same ratio, therefore, your Total Market index fund would have a lot more money in, say Microsoft, than Joe's Chicken Coup, Inc.
The Slice and Dice people, who follow the ideas of a study by Fama and French of the University of Chicago, that says smaller stocks have a larger historical return premium based on their increased risk, hold more money in small and value stocks than is represented in TSM (Total Market). The Slice and Dice camp claims the Market is overweighted in a handful of Large Growth companies (like Cisco and Microsoft), and hold that by holding more small companies, their portfolio is indeed MORE diversified.
Of course the total stock market camp says there is no better way to "hold the market" than by doing just that. But enough from me. Much Much more below...
Again, for further education, I might mention that we have some excellent articles that detail the Slice and Dice approach. Read Slice and Dice, Slice and Dice Enhanced (for international exposure), TSM/Slice and Dice article.
Also, please read John C. Bogle's first book or his recent book for not only a strong defense of Total Stock Market (TSM) funds, but also a great overall look at commonsense index investing.
Some of the posts in this thread, parts of which I may combine to make a TSM/Slice and Dice (4X25 is the same thing) debate article, could provide a strong Total Market vs. Slice and Dice education as well. If you have access to the WSJ interactive addition or that Wall Street Journal - the January 20 Jonathan Clements WS Journal article "Don't Use Index Funds As Low-Cost Sector Bets" also makes a strong case for Total Market funds.