Tenneco Inc., a major oil and natural gas company, reported record fourth-quarter and full-year sales yesterday despite a downturn in refining and marketing.

Meanwhile, Dow Chemical Co. and Pan American World Airways posted improved earnings, while Greyhound Corp. said its earnings declined.

Tenneco earned $135 million ($1.31 a share) in the latest quarter, 14 percent above 1977 fourth-quarter net income of $119 million ($1.20). Sales rose from $1.92 billion to $2.46 billion.

Profits for the full year were $466 million ($4.53), a 9 percent increase over 1977 profits of $427 million ($4.38). Sales rose from $7.41 billion to $8.76 billion.

Dow Chemical Co. reported that record sales pushed profits ahead by 3.9 percent last year and by 34 percent in the final quarter.

Dow said its net earnings were $575 million in 1978, up from $554 million in the previous year. Because the number of shares outstanding declined, per-share earnings increased 5.3 percent from $3.00 to $3.16.

The company sold a record $6.89 billion worth of goods and services last year, an increase of 10.5 percent from 1977.

Fourth-quarter profits increased from $103 million to $138 million. The increase per share was proportionately the same, 57 cents to 76 cents. Sales rose 17 percent from $1.55 million to $1.82 million.

G. J. Williams, financial vice president, said the higher profits resulted from a "robust" sales recovery starting in the summer after two weak years. The recovery has pushed plant operating rates to their highest levels in four years, he said.

Sales of agricultural chemicals were particularly strong in the United States, and sales of such plastics as polyethylene and polystyrene were so strong that shortages were appearing, Williams said.

Pan American World Airways reported profits of $2.31 a share for 1978, up from $1.06 in 1977.

Net income climbed to $118.8 million on operating revenues of $2.205 billion from $45 million in 1977 on operating revenues of $2.055 billion.

For the fourth quarter, Pan Am lost $4.6 million. This was an improvement over the 1977 fourth-quarter loss of $9.5 million. The fourth quarter is normally poor for Pan Am.

Unconsolidated subsidiaries, including Intercontinental Hotels Corp. earned $21.5 million, up from $11.5 million in 1977. But aircraft sales generated profits of $14.7 million, down from $21.6 million the previous year.

The company credited fare rate innovations for the gains in profits from airline operations.

A special charge of $51.4 million before taxes for the closing of certain Armour Food packing facilities reduced earnings of Greyhound Corp. for 1978 to $1.33 a share from $1.86 in 1977.

In addition, Greyhound had a gain of $24.78 million in 1977 on the sale of its pharmaceutical business.

Excluding the special charges, Greyhound earned $83.7 million ($1.91 a share) in 1978 against $79.4 million ($1.80) in 1977. Sales rose to $4.359 billion from $3.871 billion.

Fourth-quarter profits were 55 cents a share in 1978 on sales of $1.163 billion compared with 59 cents in the 1977 final quarter on sales of $1.037 billion.

Chairman Gerald Trautman said each of Greyhound's seven product groups was profitable during the year but earnings of the bus network were down. Service operations produced the best results, gaining 56 percent in profits.