You can even mix it:
With a 4.2 server you can still use your 3.x clients and with a 3.x server
you can use your 4.2 clients.
BUT !!! The data you backed up with a 4.2 client, you can't restore it with
a 3.x client.

So, when you upgrade your client to 4.2 and did a backup, you can't use the
old client anymore to restore it.

We did an upgrade too (3.1 to 3.7), and we started with the clients too, no
problem at all.

Maurice