Officially they provide 2000 read/write IOPS for a volume composed of one chunk (50GB) [1]. You measured 4000 r/w IOPS, so does this mean you ran it on a 100GB volume? If yes, then that's pretty amazing you get exactly the expected allocated IOPS performance for a $3.40/mo server.
[1] https://www.scaleway.com/faq/server/