Should I attempt to save 4" cast iron DWV fittings and pipe?
I am in Portland too.  Hardly anyone uses cast iron pipe anymore, so there is little demand for it.  Possibly you could donate it to the Rebuilding Center (on N. Mississippi) and take a tax deduction for it.  Otherwise, it is scrap cast iron.

There is wide variation of pricing between scrap buyers.

Bulk prices nationwide are estimated at $205 to $235 per metric ton, or $0.09 to $0.106 per pound.  It does not matter how big or small the pieces are.

None of the Portland area scrap dealers advertise their prices online.  Here is a list of contacts so you can call around and/or visit.

However, in negotiating a rate, it is better that the cast iron be relatively pure and free of mud, crud, waste, and other metals.  Hosing it down should be good enough.