CREATE TABLE #temp1 ( [field1] INT )
CREATE TABLE #temp2 ( [field2] INT, [field4] INT )
;
DECLARE @tsql NVARCHAR(MAX)
;
SET @tsql = N'
SELECT
  [field1] = [field1],
  [field4] = ' + CASE WHEN COL_LENGTH( 'tempdb..#temp1', 'field4' ) IS NOT NULL THEN '[field4]' ELSE '0' END + '
FROM
  #temp1
'
;
PRINT @tsql
;
SET @tsql = N'
SELECT
  [field1] = [field1],
  [field4] = ' + CASE WHEN COL_LENGTH( 'tempdb..#temp2', 'field4' ) IS NOT NULL THEN '[field4]' ELSE '0' END + '
FROM
  #temp2
'
;
PRINT @tsql

select
 a.n.value('f1[1]', 'int') as f1,
 a.n.value('f2[1]', 'int') as f2,
 a.n.value('f3[1]', 'int') as f3,
 ISNULL( a.n.value('f4[1]', 'int'), 0 ) as f4
from
 (select * from dbo.TempTable for xml path('row'), type) t(x) cross apply
 t.x.nodes('row') a(n);

SELECT
  a.n.value('f1[1]', 'int') as f1,
  a.n.value('f2[1]', 'int') as f2,
  a.n.value('f3[1]', 'int') as f3,
  a.n.exist('f4[1]' ) AS b,
  CASE 
    WHEN a.n.exist('f4[1]' ) = 0 THEN 0
    ELSE CONVERT( INT, NULLIF( a.n.value('f4[1]', 'VARCHAR(100)' ), '' ) )
  END as f4
from
 (select * from #TempTable for xml path('row'), type, elements xsinil ) t(x) cross apply
 t.x.nodes('row') a(n);