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.

Optimal indexes for this query:

CREATE INDEX view1_idx_id ON View1 (ID);

The optimized query:

SELECT
        DISTINCT View1_outer.ID AS PersonID,
        stuff((SELECT
            ', ' + Name 
        FROM
            View1 View1_inner 
        INNER JOIN
            View2 View2_inner 
        INNER JOIN
            View3 View3_inner 
        INNER JOIN
            View4 View4_inner 
        WHERE
            View1_inner.ID = View1_outer.ID FOR XML PATH('')),
        1,
        2,
        ' ') AS Houses 
    FROM
        View1 View1_outer 
    INNER JOIN
        View2 View2_outer 
    INNER JOIN
        View3 View3_outer 
    INNER JOIN
        View4 View4_outer 
    WHERE
        View1_outer.ID = 'XXX'