private ConcurrentDictionary<int,long> myDic = new ConcurrentDictionary<int,long>();

//Main thread at program startup

for(int i = 0; i < 4; i++)
{
  myDic.Add(i, 0);
}

//Seperate threads use this to update a value

myDic[InputID] = newLongValue;

ConcurrentDictionary<TKey, TValue>.AddOrUpdate Method (TKey, Func<TKey, TValue>, Func<TKey, TValue, TValue>);

results.AddOrUpdate(
  Id,
  id => new DbResult() {
     Id = id,
     Value = row.Value,
     Rank = 1
  },
  (id, v) =>
  {
     v.Rank++;
     return v;
  });

Parallel.For(0, 4, i =>
{
   myDic.TryAdd(i, 0);
});