If your solution is bad then anything we suggest may not be better, e.g.

SELECT b.BoxId,

MAX(CASE WHEN w.parameter = 1 THEN w.field1 END) AS [value1],

MAX(CASE WHEN w.parameter = 2 THEN w.field1 END) AS [value2],

MAX(CASE WHEN w.parameter = 3 THEN w.field1 END) AS [value3],

MAX(CASE WHEN w.parameter = 4 THEN w.field1 END) AS [value4]

FROM boxes b

LEFT OUTER JOIN widgets w ON w.boxid = b.boxid

GROUP BY b.BoxId

But the performance may be worse