using System;
using System.Collections.Generic;
using System.Linq;

namespace ConsoleApp3
{
    class Program
    {
        static void Main(string[] args)
        {
            IEnumerable<object> source = new object[] { new object[] { 1, 2, new object[] { 3 } }, 4 }; //[[1,2,[3]],4]
            IEnumerable<object> flatten(IEnumerable<object> s) => s?.SelectMany(x =>
                flatten(x as IEnumerable<object>) ?? new[] { x }
            );
            var result = flatten(source);
            result.ToList().ForEach(c =>
            {
                Console.WriteLine(c);
            });

            Console.ReadLine();
        }
    }
}