Stars and Bars representation

Here's one possibility:

With[{n = 4, k = 4}, 
     StringJoin[Riffle[Table["*", {#}] & /@ #, "|"]] & /@ FrobeniusSolve[Table[1, {k}], n]]
   {"|||****", "||*|***", "||**|**", "||***|*", "||****|", "|*||***", "|*|*|**", "|*|**|*",
    "|*|***|", "|**||**", "|**|*|*", "|**|**|", "|***||*", "|***|*|", "|****||", "*|||***",
    "*||*|**", "*||**|*", "*||***|", "*|*||**", "*|*|*|*", "*|*|**|", "*|**||*", "*|**|*|",
    "*|***||", "**|||**", "**||*|*", "**||**|", "**|*||*", "**|*|*|", "**|**||", "***|||*",
    "***||*|", "***|*||", "****|||"}

---

In a comment, Jim shows that you can use IntegerPartitions[] + Permutations[] instead:

With[{n = 4, k = 4}, 
     StringJoin[Riffle[Table["*", {#}] & /@ #, "|"]] & /@ 
     Flatten[Permutations /@ IntegerPartitions[n + k, {k}] - 1, 1]]

which should yield the same result as above.

---

The OP also wanted to consider the case where empty slots are not allowed; a slight modification of Jim's suggestion does this. Using a different example:

With[{n = 7, k = 4}, 
     StringJoin[Riffle[Table["*", {#}] & /@ #, "|"]] & /@ 
     Flatten[Permutations /@ IntegerPartitions[n, {k}], 1]]
   {"****|*|*|*", "*|****|*|*", "*|*|****|*", "*|*|*|****", "***|**|*|*", "***|*|**|*",
    "***|*|*|**", "**|***|*|*", "**|*|***|*", "**|*|*|***", "*|***|**|*", "*|***|*|**",
    "*|**|***|*", "*|**|*|***", "*|*|***|**", "*|*|**|***", "**|**|**|*", "**|**|*|**",
    "**|*|**|**", "*|**|**|**"}

★'s and |'s with string manipulations. Use Method to switch between "Positivity" (default) and "Nonnegativity".

ClearAll[starsAndBars]
Options[starsAndBars] = {Method -> "Positivity"};
starsAndBars[n_Integer?Positive, k_Integer?Positive, opts : OptionsPattern[starsAndBars]] :=
 Module[{ip = Switch[OptionValue[Method], "Positivity", {k}, "Nonnegativity", {1, k}]},
  StringReplace[{", " -> "|", "0" -> "", num : NumberString :> StringRepeat["★", FromDigits@num]}]@
   StringTake[
    ToString /@
     Flatten[
      Permutations@*Flatten@{#, ConstantArray[0, k - Length@#]} & /@ IntegerPartitions[n, ip]
      , 1]
    , {2, -2}]
  ]

Under "Positivity"

starsAndBars[4, 3] 

{★★|★|★,★|★★|★,★|★|★★}

Under "Nonnegativity"

starsAndBars[4, 3, Method -> "Nonnegativity"] 

{★★★★||,|★★★★|,||★★★★,★★★|★|,★★★||★,
 ★|★★★|,★||★★★,|★★★|★,|★|★★★,★★|★★|,
 ★★||★★,|★★|★★,★★|★|★,★|★★|★,★|★|★★}

Empty set under "Positivity" with no solutions.

starsAndBars[4, 5, Method -> "Positivity"]

{}

Hope this helps.