I found some info on CRM 4 Outlook Clients to be able to connect to a CRM 2011 server instance (after applying at least roll-up 10 or so), but is it possible the other way around?
I mean, can a CRM 2011 Outlook Client connect to an old CRM 4 server (in this case on- premise)? Our customer will have CRM 4 and 2011 servers and some employees need to be able to connect with both (if possible).
No. The new client simply will not work with CRM 4.0. Upgrade rollouts always have to be done server first, for this reason, leaving you to upgrade the outlook clients as soon as possible afterwards.
Time to get that CRM 4 instance upgraded (maybe just as a second organisation on the same CRM 2011 server you already have).