public static int[] post4(int[] nums) {

    int startPoint = nums.length - 1;
    int[] newArray;
    int amtOfElements = 0;
    int stopIndex = 0;

    for (int i = startPoint; i >= 0; i--) {

        if (nums[i] == 4) {
            stopIndex = i;
            i = 0;
        } else {
            amtOfElements++;
        }
    }

    newArray = new int[amtOfElements];

    for (int i = 0, j = stopIndex; i < amtOfElements; i++, j++) {
        newArray[i] = nums[j + 1];
    }

    return newArray;
}

public int[] post4(int[] arr) {
  for(int i = arr.length-1; i >= 0; --i) {
    if(arr[i] == 4) {
      return Arrays.copyOfRange(arr, i+1, arr.length);
    }
  }
  return new int[0]; // or null
}

public static int[] post4(int[] nums) {

    int marker = 4;
    return IntStream.range(0, nums.length) //iterate over the indexes to find the marker
                    .map(i -> nums.length - i - 1) //reverse the order to start from the back
                    .filter(i -> marker == nums[i]) //find the marker
                    .mapToObj(skip -> IntStream.of(nums).skip(skip+1).toArray()) //skip the first elements and the marker (+1)
                    .findFirst() //the new array
                    .orElse(nums); //the original array if there is no marker, or int[0]  - its not that clear from your question
}

public int[] post4(int[] nums) {

    Stack<Integer> stack = new Stack<>();
    int i = nums.length - 1;

    while (nums[i] != 4) {
        stack.add(nums[i--]);
    }

    int[] newArray = new int[stack.size()];
    int j = 0;

    while (!stack.isEmpty()) {
        newArray[j++] = stack.pop();
    }

    return newArray;
}