Hi @noel welcome to the /e/ forum.
I am pleased to hear that /e/OS installed ok for you, but I ask this really out of curiosity … to see if something changed.
While this post is from a Fairphone 3 guide [HOW-TO] Flash /e/-OS on Fairphone 3 using Debian based GNU/Linux - #58 by aibd I thought it was very similar for FP4.
adb and fastboot are included with the downloaded ROM and as long as the unzipped download remains intact and the user changes directory to work there the script is expected to find fastboot within the ROM.