manipulation of string list

First /@ 
 (GatherBy[MapAt[ToUpperCase, lis, {;;, 3}], Most] /. {{a__, "0"|"1"}, __} :> {{a, "1"}})

{{"a", "b", "X", "1"}, {"a", "f", "Z", "1"}, {"g", "a", "Z", "1"}}

Also

Tally[MapAt[ToUpperCase, lis, {;; , 3}], Most[#] == Most[#2] &] /. 
  {{a__, b_}, c_} :> {a, c /. {1 -> b, _ -> "1"}}

{{"a", "b", "X", "1"}, {"a", "f", "Z", "1"}, {"g", "a", "Z", "1"}}

and

SequenceReplace[Sort @ MapAt[ToUpperCase, lis, {;; , 3}], 
   {Repeated[{a__, _}, {2, ∞}]} :> {a, "1"}, ∞]

{{"a", "b", "X", "1"}, {"a", "f", "Z", "1"}, {"g", "a", "Z", "1"}}

This should also work:

#[[1]] /. {a_, b_, c_, _} :> {a, b, c, "1"} /; #[[2]] > 1 & /@ 
 Normal[Counts[MapAt[ToUpperCase, lis, {All, 3}]]]

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