Development tools for Zephyr on STM32Workbench for Zephyr: VS Code extension to set up, build, flash and debug Zephyr applications on STM32.Devicetree manager: browse, edit and visualize Zephyr devicetree, STM32CubeMX-like for Zephyr.
For this you should edit the link editor script (the file with extension 'ld') to have the FLASH start at 0x8004000 (changing the start address and length in the MEMORY section) then load and debug your code normally.