chore: v4.0.0 release #2579

Job	Run time
check no-default-features	14s
check serde	18s
check std	23s
clippy	1m 11s
docs	1m 21s
fmt	16s
test stable --no-default-features	37s
test no_std	45s
test stable	59s
test stable --all-features	1m 6s
test beta --no-default-features	43s
test beta	49s
test beta --all-features	1m 3s
test nightly --no-default-features	52s
test nightly	51s
test nightly --all-features	50s
	12m 18s

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