Select a repeated element in a list

You may use Tally to finish the task as follows:

Cases[Tally[list], {x_, 4} :> x]

the result will be {a,b}.

The code given by Rojo and sunt05 is almost surely the cleanest:

Cases[Tally @ list, {x_, 4} :> x]

However, here are some other possibilities:

Cases[Split @ Sort @ list, {x_, _, _, _} :> x]

Cases[Split @ Sort @ list, {Repeated[x_, {4}]} :> x]

Cases[Last @ Reap[Sow[1, list], _, {#, Tr@#2} &], {x_, 4} :> x]

Module[{c},
  c[_] = 0;
  Scan[c[#]++ &, list];
  Cases[DownValues[c], (_@_@x_ :> 4) :> x]
]

Performance

Interestingly, some of these may be significantly faster than Tally in certain cases:

list = FromCharacterCode /@ RandomInteger[15000, 100000];

Cases[Tally @ list, {x_, 4} :> x]                               // Timing // First

Cases[Last @ Reap[Sow[1, list], _, {#, Tr@#2} &], {x_, 4} :> x] // Timing // First

Module[{c},
 c[_] = 0;
 Scan[c[#]++ &, list];
 Cases[DownValues[c], (_@_@x_ :> 4) :> x]
] // Timing // First

0.546

0.109

0.2622

---

Since it seems to be only in the case of String objects that Sow/Reap is faster, for clarity one might write the second method as:

stringTally = Last @ Reap[Sow[1, #], _, {#, Tr@#2} &] &;

Cases[stringTally @ list, {x_, 4} :> x] // Timing // First

0.103

---

Addendum

The OP wrote: "I want to list all elements that appear at least four times." In light of that here are all the methods modified accordingly:

stringTally = Last @ Reap[Sow[1, #], _, {#, Tr@#2} &] &;

Cases[stringTally @ list, {x_, n_} /; n >= 4 :> x]

Cases[Split @ Sort @ list, {x_, _, _, __} :> x]

Cases[Split @ Sort @ list, {Repeated[x_, {4, ∞}]} :> x]

Cases[Last @ Reap[Sow[1, list], _, {#, Tr@#2} &], {x_, n_} /; n >= 4 :> x]

Module[{c},
  c[_] = 0;
  Scan[c[#]++ &, list];
  Cases[DownValues[c], (_@_@x_ :> n_) /; n >= 4 :> x]
]

DeleteDuplicates[Select[list, Count[list, #] == 4 &]]