well… in fact this was a misinterpretation of the log… the update frequency didn’t change, but with OH4 all item updates are reported while with OH3 only item changes were reported. That explains, that the log messages grew quite a bit.
I could solve it by fixing the log4j configuration file according to https://community.openhab.org/t/after-upgrade-to-openhab-4-a-lot-of-item-updated-to-in-the-log/147997 and https://community.openhab.org/t/openhab-4-0-milestone-discussion/145133/190. Log looks better now, superfluous messages are gone.