How to pad an integer number with leading zeros?

I have strings that are being converted that need to be four characters long. The values are coming in as anywhere between 0 and 4 characters long. I need to pad the strings with zeros to make all IDs 4 characters long:

Example

Input number Need

1  => 0001
121 => 0121
0567 => 0567

So far I have tried:

int temCode = 0; 
DataTable dt = objSites.Get_Max_SiteCode();
if (dt.Rows.Count > 0)
{ 
     string siteCode = dt.Rows[0]["SiteCode"].ToString();
     string code = siteCode.Substring(siteCode.Length - 4);
     temCode = Convert.ToInt32(code) + 1;
     temCode.ToString().PadLeft(4, '0'); // same number is coming here without any pad left.
}