walk.cs

public class Program
{
    static private Queue<long> lifo = new Queue<long>();

    static void walk(object[] root)
    {
        if (root == null) throw new ArgumentNullException(nameof(root));

        foreach (var o in root)
        {
            if (o is long i)
            {
                lifo.Enqueue(i);
            }
            else
            {
                walk((object[]) o);
            }
        }
    }


    static void Main(string[] args)
    {
        object[] srcData = {new object[] {1L, 2L, new object[] {3L}}, 4L};

        walk(srcData);

        var array = lifo.ToArray();

        foreach (var digit in array)
        {
            Console.WriteLine(digit);
        }
    }
}