The Matthews, N.C.-based company reported earnings rose 17 percent to $37.8 million, or 26 cents per share, a penny above the estimate of analysts polled by Thomson Financial's expectations.

The company said gross margin as a percentage of sales rose to 33.1 percent during the quarter ended Sept. 1 from 32.4 percent a year ago. Improved merchandise markups, a better merchandise sales mix and lower inventory shrinkage -- or reduction in recorded inventory due to shoplifting, employee theft or accounting error -- offset higher seasonal markdowns, the company said.

However, Goldman Sachs analyst Adrianne Shapira said in a note to investors same-store sales were under pressure. The company reduced September same-store sales guidance to flat to 1 percent from a previous range of 1 percent to 3 percent growth.

Same-store sales, or sales at stores open at least a year, is a key indicator of retailer performance since it measures growth at existing stores rather than newly opened ones.

"While Family Dollar's fourth-quarter margin gains and consequent earnings-per-share upside was impressive, we continue to remain concerned about the company's top-line performance," she wrote.

She advised investors to stay on the sidelines, despite an expected rise in shares due to results.

"We advise investors to avoid chasing any rally that may ensue following the company's better-than-expected fourth-quarter earnings," she wrote. "With the top line expected to remain under pressure from poor macro and competitive fundamentals, we expect continued same-store sales and earnings shortfalls to materialize."

She affirmed her "Neutral" rating on the stock.

Shares fell 86 cents, or 3.2 percent, to $26.58 in midday trading.