Plus fours may be out of style in men's fashions, but Bill Riley has fashioned his plus four into a career in the National Hockey League.

Right wing Riley parlayed a 10-game trial that began New Year's Day into a regular mace on the team; an output of 13 goals: a team-oriented aggressiveness that didn't slide into overaggressiveness, that key rating of four more goals scored by his side than the opposition while he was on ice in equal-manpower situations: his teammates' vote is the club's outstanding rookie - and now, a long-term contract with the Washington Capitals.

Riley left a full-time job as a welder in British Columbia in 1974 to pursue a most uncertain future in hockey. It became a bit less uncertain when he was signed last winter, after the tryout, for season's remainder plus an option. And, with the multiyear agreement signed yesterday, he is secure enough to say, "This is my life's business now."

Mike Marson, the only black in the NHL until Riley also joined the Caps, took a pay cut when he accepted a multiyear contract last week. You can bet Riley drew a nice raise to wear the red, white and blue Capital counterpart of plus fours (knickers to you, young fella).