Hi I have a question
Does arc 4 work even without purchasing the arc studio device?
Or must also purchase an arc studio device in order to work with it.
You need some way to apply the corrections calculated by ARC. The usual way is to use the ARC VST plug-in to apply them in your Digital Audio Workstation (DAW) software. It is possible to use another VST host like Voicemeeter Banana to apply the corrections and do this systemwide. I have done this with ARC3, but it's a bit fiddly and tends to get broken (and require re-setup) after Windows updates. (note: eAPO can also act as a VST host, but I couldn't get it to work with the ARC3 VST).
The other issue I had with ARC3 is that it requires 27 measurement positions for the mic. This is very tedious to execute. Personally, I don't think it's worth the effort for music listening only.
I own ARC3 because I could buy it a discount as an iLoud MTM owner. It was fun to play with, but I don't use it anymore. My current system was setup using the moving mic method with a UMIK-1 & REW, with PEQ applied system-wide using eAPO - it's a lot quicker and easier than using ARC.