I started my career at my current company, and have four years wirth of relationships built across various depratments. I've taken another job and given notice to my manger.

What's the best way to share my plans, contact information and say goodbye with these contacts to make sure that they stay in my network within the context of jumping ship?

Bender13

Resignation e-mail stating your enjoyment with working with everyone and your home e-mail address. Broadcast though the company, you might have contacts you didn't know you had.

Example
I just wanted to say goodbye and a very sincere and big thank you to all of the people that I have had the pleasure to work with during my years with xxx. It has been a great experience and I have been very fortunate to have worked with some fantastic and very professional people within xxx. If you would like to keep in contact my e-mail address is xxx

IM4U

Oops, didn't check the date before I rated the answer below. But I still think it's a good answer.