A RETURN LAND BY A BE24 SIERRA CFI WHEN HE RAN INTO FORECAST ICING CONDITIONS AT 4000 FT 20 W OF BVI; PA.

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I DEPARTED IFR FROM BVI EXPECTING TO BE ABLE TO AVOID FORECASTED ICING CONDITIONS BY REMAINING CLR OF CLOUDS AND PRECIP. WITH AN AIRMET FOR ICING FROM 4000-12000 FT FOR MY RTE OF FLT; I BELIEVED I COULD AVOID ICE BY STAYING AT 4000 FT UNTIL I GOT TO WARMER AIR. UPON CLBOUT; I NOTICED THAT THE OAT WAS -5 DEGS CELSIUS AT 4000 FT. I HAD EXPECTED IT TO BE ABOVE FREEZING AT THAT LEVEL. UPON ENTERING IMC CONDITIONS; I STARTED TO PICK UP LIGHT MIXED ICING. PITTSBURGH DEP ADVISED ME THAT THE TOPS WERE 9000 FT. I IMMEDIATELY ASKED TO RETURN TO BVI AND A DSCNT TO 3000 FT. AT 3000 FT; THE ICE MELTED. I RETURNED TO BVI WITH NO FURTHER ICE ENCOUNTERS. HAD I CONTINUED THE FLT; I COULD HAVE HAD SERIOUS PERFORMANCE PROBS. FLT IN OR CLOSE TO AREAS OF FORECAST ICING CONDITIONS CAN BE VERY DIFFICULT IN UNPROTECTED ACFT. I BELIEVE MY 'OPTIMISTIC' OUTLOOK AT THE AIRMET FOR ICING MADE ME BELIEVE THE FLT WOULD BE UNEVENTFUL. PLTS SHOULD LOOK AT ALL FORECASTS WITH A SKEPTICAL VIEW AND DETERMINE WHAT WE WILL DO WHEN THE WX IS WORSE THAN FORECAST.