COLUMN 1 | COLUMN 2 |  COLUMN 3 | COLUMN 4
Hallo      out         there      Hallo out there
My         NULL        name is    My name is
I'm        a           rabbit     I'm a rabbit

UPDATE dbo.table
  SET column4 = COALESCE(column1, '')
      + COALESCE(' ' + column2, '')
      + COALESCE(' ' + column3, '');

select ISNULL([COLUMN 1],'')+' ' +
         isnull([COLUMN 2],'')+' ' +
         isnull([COLUMN 3],'')
  from your_table

UPDATE table
SET Column4 = CONCAT(Column1 + ' ', Column2 + ' ', Column3)

UPDATE table
SET Column4 =
ISNULL(Column1+' ','') +
ISNULL(Column2+' ','') +
ISNULL(Column3,'')

select isnull(convert(varchar(255),[COLUMN1]),'')+' '+isnull(convert(varchar(255),[COLUMN2]),'')+' '+isnull(convert(varchar(255),[COLUMN3]),'')
from table

UPDATE table
SET COLUMN4 = isnull(convert(varchar(255),[COLUMN1]),'')+' '+isnull(convert(varchar(255),[COLUMN2]),'')+' '+isnull(convert(varchar(255),[COLUMN3]),'')

update yourtable SET COlumn4= ltrim(rtrim(isnull([COLUMN 1],'') +' '+isnull([COLUMN 2],'') +' '+  isnull([COLUMN 3],'')))