Update: 18th May Amazon have a special price for the Season 4 DVD's if you pre-order them. $38.99 for the DVD and $66.95 for the Blu-Ray.
The Season 4 DVD and Blu-Ray Releases for Season 4 are now both available for pre-ordering.
USA Version - DVD
USA Version - Blu-Ray
UK Version - DVD
UK Version - Blu-Ray
Canadian Version - DVD
Canadian Version - Blu-Ray
Thanks to Silent Impression for the Canadian Links