I’m assuming that you want to do this migration exactly once and ditch the old WIndows domains afterwards. For this you can use the Univention ActiveDirectory Connector. It can implement a one-way synchronization of users and groups from a Windows domain to a UCS domain.
So for your case I’d suggest you install the AD connector, configure it against the second AD domain, and let it sync. Then you re-configure the AD connector against the third Windows domain and let it sync once more. Finally repeat this for the fourth AD domain. Afterwards the AD connector can be uninstalled again.
The result should be a UCS domain that contains all users and groups of the four former AD domains.
Note that name conflicts may cause problems, e.g. the members of well-known groups like Administrators will probably be overwritten by subsequent syncs.
I haven’t implemented such a scenario yet, but it should work.
What you cannot do in such a scenario is migrate the computer accounts from the other three domains if I’m not mistaken.