Hi Stephen,
Looks like a great ride!
I grew up in the Lost Coast area (Whale Gulch), so your first route goes right through my home turf.
One thing that I find suspicious is that RWGPS seems to show some paved road between Four Corners and Dutchman's Flat, and I don't believe that's true. The Usal Road is pretty remote and depending on the time of year, not well maintained. I believe the county only maintains it during summer months, so in winter it is sometimes unpassable in a car. That shouldn't stop a bicycle, but you may find areas pretty rutted. YMMV! I've ridden it only once, in summer, and my problem was not bringing enough water, so plan accordingly. It's only about 26 miles, but as you can see from RWGPS, it's a pretty intense 26 miles! Don't let that scare you off, just be prepared. You will be rewarded by some spectacular country. You can probably fill water bottles at the Whale Gulch School, just before you get to Four Corners. After Four Corners, you'll be heading off into truly unpopulated areas and are unlikely to find much water. (Filter what you do find!) Oh, and if it is dust season (June-Sep), you may want to have a bandana around your neck, ready to pull up over your mouth/nose for when cars (very occasionally) pass. Ah, I just saw that you're thinking of April, so dust will not likely be an issue, but rain may be; you'll be riding through one of the wettest regions in the whole state! ;-)
https://www.eldoradoweather.com/calprecip-full-size.htmlHave a great ride, and I look forward to your ride report...
cheers,
Andrew, from Whale Gulch; now in Sydney
(Image from Google Street View at Four Corners)