Redwood Backpack

You might look into the California Coastal trail from Patrick's Point State Park to the Oregon border. Obviously that's more than 3-4 days but gives you a lot of possible options. Unfortunately, due to closures the options are far fewer than they were 10 years ago.

One option would be to start at the parking area for the Tall Trees Grove in Redwood National Park. You can camp along Redwood Creek North of the Footbridge.  They may have changed this though and maybe they now force you to stay in the crappy Elam Camp or crappier 44 Camp.

The next day would be a long day down to Bald Hills Rd, across to the Skunk Cabbage Trail to the Pacific before a 2 mile dirt road walk to Gold Bluffs. Gold Bluffs is extremely popular so if there aren't any reservations left, there's a hiker/biker site you can share.

There used to be an Environmental Camp and Miner's Bluff but they're both closed (even though the land itself doesn't disappear).

From Gold Bluffs, you used to be able to hike to Ossagon / Butler Creek, but they closed that site as well. Instead, consider hiking via Fern Canyon to Elk Prairie. Elk Prairie is also popular but it also has a hiker/biker site.

A really long alternative to Elk Prairie would be to go to Flint Ridge, but it's another long-ish road walk, although Ossagon Rocks are really cool.

Further North is DeMartin Camp. 10 sites but I've never seen anyone in any of them. The hike from DeMartin to Nickel Creek (also hard to get reservations) is really spectacular through old growth redwoods.

There's not much camping North of Crescent City and the trail is mostly along the beach. They closed Tolowa Dunes a few years back so that's a drag (even if it wasn't the most amazing camping ever).

I've never been to Jedediah Smith, but it appears that it's not well suited to backpacking.
Steve