I have about 500' of 4-year-old cedar fence, with 4x4 posts sunk in
concrete foundation posts. Some of the fence has been beaten a little
with trimmers, etc, and it appears quite weak and brittle. Do I need
to seal it? Pressure wash it first? Or just leave it? For now its ok,
but I want it to last me 20-30 years. Are people treating fences like
decks here on this group?

See my comments several threads above, on preserving Rough Sawn Cedar.
Cedar will gradually fade to a silvery gray, and sort of stabilize on its
own. Staining it will add some cosmetic value, but not necessarily stop slow
rotting and splitting. I personally like the silvery gray it turns, with no
staining.
Key area is underground. When fences fail, they either blow down, or their
posts rot through at or below the soil line, where the post is always damp,
as it is subject to dry rot, fungus, and termites. I just wait till I can
wiggle a post, and see that it is clearly weak or loose or rotted. Then I
saw off the post near the base, break up the concrete with a punch bar, or
pry it out of the ground, and replace it with pressure treated or synthetic
wood, and leave lots of crushed 3/4 inch gravel in the base of the hole for
drainage. One can also twin the posts, avoiding digging the concrete out.
Bottom line: I would just leave it, and wait, and check posts now and then.
Roger