I really liked the web-installer, however I think there should have been a note on the website about re-locking the bootloader is necessary and requires additional tools (as pointed out above). I only realized what all this means because the web-installer under windows failed on my FP4 and I reinstalled Fairphone OS. After @mnd 's and @Immiperium encouragement i will attempt web-installer now with Brave browser and will report back whether it worked. Thanks you two for sharing your experiences!