NumericExtensions.cs

using System;
using System.Text;

namespace NumberToAlpha
{
    public static class NumericExtensions
    {
        public static string ToAlphabetic(this int number)
        {
            if (number < 1)
                throw new ArgumentOutOfRangeException("number");

            var minCharCode = Convert.ToChar("A");
            var lengthOfAlphabetics = 26;

            var capacity = (int)Math.Floor((double)number / lengthOfAlphabetics) + 1;

            var sb = new StringBuilder(capacity);

            var currentCharCode = number;
            var prevCharCode = 1;

            var charCode = -1;

            while (currentCharCode > lengthOfAlphabetics)
            {
                if (prevCharCode > lengthOfAlphabetics)
                    prevCharCode = 1;

                charCode = minCharCode + prevCharCode - 1;

                sb.Append((char)charCode);

                currentCharCode -= lengthOfAlphabetics;
                ++prevCharCode;
            }

            charCode = minCharCode + currentCharCode - 1;
            sb.Append((char)charCode);

            return sb.ToString();
        }
    }

}

Program.cs

using System;

namespace NumberToAlpha
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Calculating...");

            var alphabetics = 4000.ToAlphabetic(); // int.MaxValue.ToAlphabetic();
            Console.WriteLine(alphabetics);

            Console.ReadKey();
        }
    }
}