I can believe the elonation being imperceptable over 12" after 4500 miles, but not the .210 roller spacing. I'd say your caliper tips are not hitting the center of the roller. I would expect a spacing around .215 after 2,000 miles or less and maybe .230 after 4500.
If that's the first chain on a cassette, I'd be getting it off quick, unless you intend to use that one chain for 10,000 miles and trash both the chain and cassette together. You may already encounter chain skip on your most-used cog, if a new chain is installed now. With 10 speed, I alternated chains every 2500 miles, so the third and last chain in the rotation went on a 5,000 miles, just to insure that I got no chain skip.
I just checked my work by remeasuring more thoroughly. I measured between rollers (between outer plates where there's more width) at several links along the chain 3x each, making sure the caliper tips were hitting center and the rollers were being displaced away from each. I'm getting .214" consistently. I also did a plug test using a flat tip screwdriver that is .210" at the tip and is marked at the .230" wide point (.280" down from the tip). That test result appears to be consistent with my caliper measurements and certainly shows that the spacing is not even close to .230".
If I had a willing and able assistant to take pictures to show how I do my measurements I would but I don't.
Anyway, I plan on continuing to use this chain and cassette until the spacing reaches at least .230" and then replace both. Once there is a purpose made 11sp master/quick link out on the market with a good established reputation I'll switch to the 3 chain to 1 cassette rotation system.