Do you have any insights on resource needs of the hardware? One thing I ALWAYS find lacking in documentation of fediverse software is hardware specs for e.g. 1 user instances (as a basic benchline).
For a very small instance, I'd look at Pleroma. (Disclaimer: I've never installed it, so this is hearsay.) My understanding is that it runs fine on 1GB RAM instance, and by default doesn't cache media locally so you don't have to have tons of drive space. Also, a single user instance will only have to have enough DB storage to track the toots of the users you follow, so unless you immediately log in and follow 4,000,000 users that shouldn't be a bottleneck either.