If you are planning to take Route 6 to access this cache you should be aware of a steep narrow section for approxiamtely 1 mile. The road is very drivable, but it is bumpy, narrow AND gets steep. It is taking you to the South Fork of the Trinity River!! Just take your time. There is so much Forest to explore! Check out a map.
You do not need a 4x4. A Geo metro passed me while I was out here. It is just up to you and your comfort level; driving and for your car.
You can also reach this cache via Hyampom and Trinity County Rd. 311
I really suggest you bring a forest map with you. You can get one here http://www.fs.usda.gov/main/srnf/maps-pubs. You will not have cell coverage in most of this area.