OK, If that's what he wanted the rosito's first reply would nearly have got him that, all he needed to do was trap the "not a number" bits. e.g.
=if(isnumber(a1),if(a1=1,"A",if(a1=2,"B",if(a1=3,"C",if(a1=3,"D","Error !")))),"")
or
=if(isnumber(a1),mid("ABCD",find(a1,"1234"),1),"")
Depending on...