Using your exact code (and renaming the table) this is what I got from Access:
Select tblA.fld1, tblA.fld2, tblB.fld3, tblB.fld4 From tblA Left Join tblB on tblA.fld1 = tblB.fld3
Union Select tblA.fld1, tblA.fld2, tblB.fld3, tblB.fld4 From tblA Right Join tblB on tblA.fld1 = tblB.fld3...