Is the son sinful if he refuses to accompany his mother for Umrah for the fourth time, knowing that he has financial obligations for building his house and that the sheikh advised him to go for Hajj instead of Umrah?

Your mother is obligated to perform the Hajj if she is able and financially capable, as Allah Almighty says: "And [due] to Allah from the people is a pilgrimage to the House - for whoever is able to find thereto a way. But whoever disbelieves - then indeed, Allah is free from need of the worlds." As for Umrah, she has performed it more than once, so it is not obligatory upon her. If your mother is unable to afford the Hajj, she may perform Umrah. As for her request for you to accompany her at your own expense, you are not obligated to obey her if you are not financially able, and you will not be sinful if you kindly apologize to her.