The optimization process and recommendations:

1. Create Optimal Indexes (modified query below): The recommended indexes are an integral part of this optimization effort and should be created before testing the execution duration of the optimized query.
2. Replace In Subquery With Correlated Exists (modified query below): In many cases, an EXISTS subquery with a correlated condition will perform better than a non correlated IN subquery.

Optimal indexes for this query:

ALTER TABLE `Table` ADD INDEX `table_idx_id4_columnn1` (`id4`,`Columnn1`);
ALTER TABLE `Table2` ADD INDEX `table2_idx_id2_id3_id` (`id2`,`id3`,`id`);

The optimized query:

SELECT
        COUNT(*) 
    FROM
        dbo.Table WITH (NOLOCK) 
    WHERE
        dbo.Table.Columnn1 IN (
            1, 2
        ) 
        AND EXISTS (
            SELECT
                1 
            FROM
                dbo.Table2 
            WHERE
                (
                    dbo.Table2.id2 = 5 
                    AND dbo.Table2.id3 = 1
                ) 
                AND (
                    dbo.Table.Column2 = dbo.Table2.id
                )
        ) 
        AND dbo.Table.id4 = 8