You'll need to know the size of the contactor as well as the model to know what parts may be available. Contactors range in size from 00 to 7. Here's a link to a NEMA chart showing the various sizes, amperage ratings and voltage ratings. Once the size is determined you can do a search on eBay for potential replacement parts.
Nema sized contactors or starters.
www.engineeringtoolbox.com
If yours is i size 00 or 0 complete new contactors and most parts are often be available a reasonable cost. As an FYI Furnas has changed hands several times over the years and is currently owned by Siemens. I have been able to find new take off contactors for my older size 0 Furnas starters for only a few dollars. I'm not familiar with the R4 series, but all contactors I have had experience with can be disassembled to replace the contactor points and related hardware.
I found new magnetic coils for less than $20.00 each, and contacts between $13.00 and $18.00 per set.
Another option would be to change out the complete contactor/starter combination for an Allen Bradley setup. I was able to buy a complete NOS control cabinet with relays, reversing starters, including the magnetic coils and contactors for $99.00. If or when the Furnas controls die, or I can no longer find replacement parts at a reasonable cost I'll swap out the complete system.