You probably want to do the grouping in the inner query rather than the outer query. Also, can limit parameter values to 1-4 in the inner query itself.

SELECT a.BoxId, b.value1, b.value2, b.value3, b.value4

FROM boxes a

LEFT OUTER JOIN (

SELECT boxid,

MAX(CASE WHEN parameter = 1 THEN field1 END) AS value1,

MAX(CASE WHEN parameter = 2 THEN field1 END) AS value2,

MAX(CASE WHEN parameter = 3 THEN field1 END) AS value3,

MAX(CASE WHEN parameter = 4 THEN field1 END) AS value4

FROM widgets

WHERE

parameter BETWEEN 1 AND 4

GROUP BY boxid

) AS b ON a.boxid = b.boxid