The "delete" key does not work in TS 4.0.0. It simply does nothing.
My environment: OpenSUSE 15.3, package texstudio-4.0.0-lp153.10.1.x86_64 from, distribution: home:jsundermeyer / openSUSE_Leap_15.3.
(Sorry, first I forgot to login, and now I repeat the bug to be notified about comments.)