At the time of support, our technicians would require read and write access.
As previously mentioned, you would need to be fully on the version 4 platform. The upgrade and it's background tasks need to be completed. However, if at time of support, it is deemed that poor data from version 3 is causing an issue, that may be outside our scope of support. That is entirely rare but for instance, if a plugin from version 3 corrupted core data and got upgraded to version 4, that is not something which we would support. As well, for instance, if the wrong collation was used during the upgrade of version 3 data resulting in bad/corrupted data in version 4, the instructions would be to re-do the upgrade with the right collation, etc...
Unfortunately, I cannot provide you with all cases as mentioned, it would be highly case-by-case but these are some examples. If things are looking good with your upgrade, I would not even bother worrying about it 🙂 .