dataset = Dataset[Table[<|"a" -> n|>, {n, 3}]];

dataset[All, Range[4][[#a]]&]

dataset[All, Range[4][[#a + 1]]&]

dataset[All, Extract[Range[4], {#a + 1}]&]

ds = Dataset[Table[<|"a" -> n|>, {n, 3}]];
 ds[All, "a", # + 1 &]

ds[All, "a"]