exchange hybrid multiple forests single email domain

The good news is it sounds like you have already done the hard part, which is getting AAD Connect to sync all objects from 4 different Forest/Domains to a single O365 tenant.

Establishing Hybrid is simple in your case because you only have a single Exchange on-premises server.

If you had a separate Exchange Server in each forest it would have been more complex.

The Exchange Hybrid wizard in your case will be mostly the same as if you had a single forest with a single Exchange server because you have already done the hard part of doing the Identity sync with AAD Connect.

So my advice is to follow the Exchange Deployment Assistant here:

https://technet.microsoft.com/en-us/office/dn756393.aspx

I assume that your AAD Connect was setup with this supported topology document in mind:

https://docs.microsoft.com/en-us/azure/active-directory/connect/active-directory-aadconnect-topologies

Review Hybrid Prerequisites here:

https://technet.microsoft.com/en-us/library/hh534377(v=exchg.150).aspx

One thing that you may run into that is not well documented anywhere is creating new accounts in those separate AD forests after hybrid is established. You'll need to create them as you normally would, then migrate those mailboxes to O365. Therefore, this requires you to maintain your Exchange Hybrid server until Microsoft resolves that issue (During the Ignite conference MSFT announced a future feature that will improve this scenario). Currently, there doesn't seem to be any way to create a linked mailbox directly in O365. For example, https://technet.microsoft.com/en-us/library/ff607480(v=exchg.160).aspxor enable-remotemailbox doesn't seem to support a linked mailbox scenario.

Feel free to reach out to me directly if you need additional guidance:   joe@patriotconsultingtech.com