THE PILOT REPORTED THAT HE ENCOUNTERED A SNOW SQUALL WHILE CRUISING AT 5000 FT MSL ON A VFR FLIGHT PLAN AT NIGHT. HE STATED THAT HE DESCENDED TO 4000 FT MSL TO TRY TO GET UNDER THE SQUALL AND CONTINUE. HE WAS UNABLE TO REMEMBER WHAT HAPPENED AFTER DESCENDING TO 4000 FT MSL. THE PLANE WAS FOUND AND THE SURVIVORS WERE RESCUED WITHIN TWO HOURS AFTER THE ACCIDENT. THE AIRCRAFT HAD CRASHED ON LEVEL, SNOW COVERED TERRAIN AND CONTINUED ABOUT 300 FT AFTER IMPACT. AN INSPECTION OF THE WRECKAGE REVEALED NO PREIMPACT MALFUNCTION OR FAILURE OF THE FLIGHT CONTROLS OR ENGINE. LOCAL RESIDENTS ESTIMATED THE VISIBILITY FROM LESS THEN ONE MILE TO THREE MILES WITH SNOW FALLING AND BLOWING. THE WIND CHILL FACTOR WAS ABOUT MINUS 50 DEGREES FAHRENHEIT. THE PILOT WAS NOT CERTIFICATED TO FLY AT NIGHT. THE ELEVATION OF THE CRASH SITE WAS ABOUT 2200 FT MSL.