(CRM 4) Accessing product entity from product lookup in opportunityproduct(javascript)

Hello,

I think this answer will be fairly straightforward for most of the CRM users:

How can I access all fields from the product entity in the onchage event of the product lookup when I'm creating a new opportunity product?

I'm aiming at doing this through Javascript and the reason I need it is because I need a value from a field in that entity to populate the form whenever a user changes a the product.

I already have the productid (that's one of the few fields the lookup has) but I'm not sure how to get the whole record associated with it.

I'm using CRM 4 not 2011.

Thanks.