Welcome to Marriott Rewards Insiders (MRI). The one property that immediately comes to mind that would fit what you are looking for is Fairfield Inn & Suites Jacksonville Beach, Jacksonville Beach Hotels. You can use the "Reviews" section of this website to see what others have said about the property.
pluto77 has written extensively on MRI about California. She may know of any properties that meet what you are looking for in California.
Wherever you go, enjoy your travel and come back to the site and tell us about it.
I tried this exercise a couple of years ago (even before the Category increases) and you're right, it's pretty disappointing. For ol' times sake, I went to advanced search, clicked on hotel directory, went to Florida, clicked on Hotel Category #4 and came up with 67 properties, most of them west of Rt. 1 or east of the gulf.
The Fairfield Inn in Jacksonville says it's 100 yards from the white sandy beach. Trip advisor is so-so (some saying there's nothing else) but for a Cat. 4 and beach, it looks like a possibility.
California used to have the Fairfield in Capitola - I never made it and it's now a 5 (probably on its way to a 6).
Yes, since the big category bump last May, it is almost impossible to find category 4 hotels near the water in CA, as they are all cat. 5 and above now. I suggest that you contact Marriott customer service and ask to be put into the category 5 certificate mega bonus promotions in the future, which will help some, but still will be a challenge to find places in CA within walking distance to the beach (though you can find places that are only a short drive.) For the cat. 4's however, you might have to accept the fact of a 10 mile drive to the beach, say from Irvine, Santa Ana or Costa Mesa (to Newport Beach) or from San Marcos (to Carlsbad by the Sea.) If you need any other ideas for those cat. 4 cents, let us know.