This green caterpillar is about 3.5-4″ long. Appears to have one set of legs at the end, 4 sets in the middle and another 2 sets up front (maybe?). Has 6 black/yellow balls (maybe eyes or sensors), and a little yellow pokey thing at its other end. (Valley Center, Ca, USA)
Your caterpillar is a Ceanothus Silkmoth larvae. It turns into a beautiful huge moth.
Here are photos of the moth and some of the other larval stages. This caterpillar looks different at different stages. The yellow and black balls are just spines on the back of the caterpillar and the yellow knob at the tail end is just that. It has 6 legs in front, then 4 sets of prolegs and then the rear 2 legs. http://bugguide.net/node/view/42242/bgimage?from=24
With the size of your caterpillar it is in the late stage and should start pupating soon.
This insect is found from British Columbia to western Montana, south through west coast states to Baja California. In California, it is found mostly west of the Sierras.
The caterpillar feeds on a number of trees and shrubs, including Red Alder, birch, Antelope Bitterbrush, buckthorn (Rhamnus), Buffaloberry, Ceanothus species, cherry, gooseberry, Hardhack, hazel, Madrone, manzanita, Mountain Mahogany, Mountain Maple, rose, Saskatoon Serviceberry, willow, and occasionally Douglas-fir.
Moni has an ebook “Garden Insect Pests of North America – Pictures for Identifying and Organic Controls” Check it out!