Grenadas... how to tell what year?

Did granada switch to 4 lug in 1980, or 81?

No/Yes.

The Ford Granada/Mercury Monarch had the same body style from 1975 through 1980. --The '77-'80 Lincoln Versailles was basically the same as the Granada/Monarch, only a little fancier and with a 'hump' on the trunk lid. All had 5-on-4½" inch lug patterns [same wheel pattern as '65-'73 & '94-present Mustangs].

The Granada didn't change body styles until 1981. It actually took on the Fox platform --same as for the Ford Fairmont/Mercury Zephyr, T-Bird and the Fox Mustangs of the era. All these bodies had 4-on-4¼" lug patterns.