You may disable constraint like that

alter table dbo.T4 nocheck constraint fk_T4_sid

and do what you want, but, be aware of performance problems. I blogged about it here[/url] (unfortunatelly it is only in russian, but t-sql and plans talk for them selfes).

To have all the benefits of FK, your FK should be trusted, that means, that there should be a solid integrity of data and no missing values of fk column for pk.