International Business Machines Corp. yesterday reported sharply higher earnings for the fourth quarter, but Wall Street analysts say they were disappointed in the results because much of the dramatic jump could be attributed to nonrecurring gains.

The Armonk, N.Y., computer giant reported net earnings for the quarter ended Dec. 31 of $2.1 billion ($3.47 a share), up 50 percent from $1.4 billion ($2.28) in the year-earlier quarter. Revenue was up 6 percent, to $18 billion from $16.9 billion a year earlier.

For the full year, IBM had net earnings of $5.3 billion ($8.72), up 10 percent from $4.8 billion ($7.81) in the previous year. Revenue was up 6 percent, to $54.2 billion from $51.3 billion in 1986.

"These results demonstrate that our continuing efforts to make IBM more competitive are beginning to pay off," said IBM Chairman John F. Akers.

But industry analysts and investors did not seem to share his belief that IBM had emerged from recent hard times, even though the company was able to break its two-year streak of falling earnings.

Most analysts had predicted that IBM would post fourth-quarter earnings in the $3.25- to $3.75-a-share range, with many at the higher end.

They expressed strong concern that IBM's earnings, while within that range, benefited significantly from such nonrecurring items as a less-than-expected tax rate. IBM said its tax rate for the fourth quarter was 36.4 percent, compared with 45.3 percent in the fourth quarter of 1986. Many analysts say they had built a 41 to 42 percent tax rate into their projections.

"The real source of disappointment is the revenue picture," said Michael Geran, an analyst at Nikko Securities in New York. "Domestic revenues were down. So much of the gain was the result of currency translations."

Some estimates put the amount of fourth-quarter earnings that can be attributed to currency changes as high as 50 cents a share, with another 25 cents a share due to the lower tax rate.

"The gains based on currency changes are something that you can't depend on being repeated -- the dollar can't go much lower than it's been this quarter," said Julian Menear, an analyst at Pershing Co. in Chicago.

Also creating a favorable impact on earnings were such one-time events as gains on the sale of Intel Corp. stock and Intel bond redemptions, and a corporate stock buyback program, which resulted in 7.4 million fewer shares of stock outstanding in the final quarter of 1987 compared with 1986.

The disappointing earnings report came just a week after two of IBM's biggest competitors announced an alliance that could strengthen their positions against the nation's leading computer maker.

Apple Computer Inc. and Digital Equipment Corp. signed a technology agreement last week designed to allow Apple's personal computers and Digital's minicomputers to work together.