In MongoDB.Bson.BsonDefaults,
private static int maxDocumentSize = 4 * 1024 * 1024; // 4MB

should be:
private static int maxDocumentSize = 4 * 1024 * 1024 * 4; // 4MB * 4 = 16MB

Otherwise, during a Read, the ReadSize method throws an error:

private int ReadSize() {
int size = buffer.ReadInt32();
if (size < 0)

{ throw new FileFormatException("Size is negative"); }

if (size > settings.MaxDocumentSize)

{ throw new FileFormatException("Size is larger than MaxDocumentSize"); }

return size;
}