Assuming that the Bag class is implemented efficiently using a fixed array with a capacity of 4000, we execute the following statements: ```cpp Bag b; b.insert(5); b.insert(4); b.insert(6); b.erase_one(5); ```

After executing the given sequence of statements, let's break down what happens at each step in relation to the `Bag` class:

1. Bag b; - This line of code declares a variable `b` of the type `Bag`. It's an instance (object) of the class `Bag`. Assuming that the `Bag` class constructor initializes the internal fixed array to an empty state, at this point the `Bag` is empty, with no elements stored.

2. b.insert(5); - The `insert` member function is called on the `Bag` instance `b`, requesting to insert the element with the value `5` into the bag. The `Bag` will place the number `5` into the next available slot within its fixed array.

3. b.insert(4); - This statement calls the `insert` function again, this time with the value `4`. The number `4` is then placed into the next free slot in the bag's internal array, following the number `5`.

4. b.insert(6); - The `insert` function is called for the third time, adding the number `6` into the next free slot within the bag's internal array. Now, the bag contains the elements `5`, `4`, and `6`.

5. b.erase_one(5); - Finally, the `erase_one` member function is called to remove one occurrence of the element `5` from the bag. Since the `Bag` class is implemented efficiently, this operation will likely involve finding the element `5` in the array, remove it, and then potentially shifting subsequent elements to fill the gap to maintain the array's contiguous nature or marking that slot as available for future insertions. This would depend on specific implementation details.

After all these statements execute, the `Bag` would contain the elements `4` and `6` (assuming the `insert` and `erase` functions work as intended and no other member functions are called between these operations).