Twitter Inc., the microblogging service that held its initial public offering last month, rose to a record for a fourth trading day amid optimism that the company has room to expand sales in digital advertising.

Near midday, Twitter was up $3.99, or 5.7 percent, at $73.95. Twitter jumped 8.4 percent Tuesday after rising 7.5 percent Monday. The stock has surged more than 170 percent since going public at $26 on Nov. 6, giving Twitter a market value of almost $40 billion — larger than Time Warner Cable Inc., media company Viacom Inc. and retailer Target Corp.

Investors are betting San Francisco-based Twitter can attract more ad dollars as companies seek to market their wares through wireless devices and tie-ins with television programming.

As the unprofitable company’s sales growth outpaces that of competitors such as Facebook Inc. and Google Inc., the stock has “graduated to cult status,” according to Blake Harper, an analyst at Wunderlich Securities Inc. in Baltimore.

“While the company is growing revenues faster than its fastest-growing peers and we do recognize the potential for the company to capture larger portions of the mobile and TV advertising market, it appears valuation metrics are irrelevant and that investors are betting aggressively on Twitter being the next great media-technology platform,” Harper, who recommends selling the stock, wrote in a note today.

“We don’t believe the current valuation justifies the risk,” wrote Harper, whose price target for Twitter is $34. “Especially with the company having yet to report a quarter as a public company.”