Like you said yourself, Nextcloud can be hosted on a PI so this should do.

You can see the hardware requirements here : https://docs.nextcloud.com/server/latest/admin_manual/installation/system_requirements.html

The CPU supports 64bit and your 4GB memory is way above the 128mb minimum.

If you aren’t using that PC for anything else, I would just go a head and try and see how it works out.

Why don’t you tell us what you want to do with it, ‘self hosted cloud storage’ is a bit vague.

You can probably run all kinds of projects on it for personal use but it might struggle with multi user media streaming, for example.