Legg Mason to Offer 4 Million Shares  NEW YORK (Reuters) - Legg Mason Inc. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=LM.N target=/stocks/quickinfo/fullquote"&gt;LM.N&lt;/A&gt;, one of the  fastest-growing U.S. asset management companies, on Wednesday  said it plans a public offering of 4 million shares, a sale  that could raise almost \$300 million.