Microsoft Access: What Is The Command Syntax To Replace Every 4th Character With A Space?

Use the Left( and Right( string functions to select the spaces to the left and to the right of the fourth character. The join them together.
ILen = Len(MyString) - to find the length of the string
strLeftstring = Left(MSstring, 3) - to select the three characters to the left of the fourth character
strRightstring = Right(MyString,(iLen - 5) ' start one character to the right
strNewString = strLeftString + " " + strRightString

Update into the database
rs1!FieldNameToUpdate = strNewString
rs1.Update