I know I skipped season 3 but oh well :3

~Senario~

Izzy died 2 years ago and sam died a year later.

Dean was depressed and decided to sell his soul to get them back. He did but only got Sam back and he only had a year

~~Beginning of season 4~~

Dean was back and Izzy was still 'dead' until they were on a hunt for a demon. But they didn't know the demon was Izzy.