If you want to avoid nested IF fields, simply put them one after another:
{ IF {REF One} = "One" "1" "" }{ IF {REF One} = "Two" "2" "" }{ IF {REF One} = "Three" "3" "" }{ IF {REF One} = "Four" "4" "" }

Otherwise, you can put the successive fields inside the quotemarks for the false text.
{ IF {REF One} = "One" "1" "{ IF {REF One} = "Two" "2" "{ IF {REF One} = "Three" "3" "{ IF {REF One} = "Four" "4" "" }" }" }" }
I believe you can go 16 deep.