For more than 1 country, you need a global pass. But also compare normal tickets with a pass and reservations.

Depends on the number of days you plan to travel for. The swiss pass can get expensive and sometimes costs as much as an eurail pass. Also the flexibility of the global pass is great if you use your travel days judiciously. 

I agree with @rvdborgt - compare normal tickets + seat reservations against the pass and then decide.

We will arrive Milan and straight to Rome for 4 days, to Florence 4 days, to Lucerne 3 days and then back to Milan. Should we buy Eurail pass or just buy the ticket when avaliable?

Switzerland: normal fares are high, no reservations needed, a rail pass is great.

Italy: normal fares can be very cheap, Eurail pass-holders need to pay reservation fees = a rail pass isn’t so good here. It will cost you €10 per train on the fast trains, €3 on slower intercity trains, €11 on the international train. 

Thanks Rladerton for your reply. Should I buy a 4 days pass?

You can only determine that yourself. Look up the normal fares for your journeys and be aware that fares are lower when booking a longer time in advance. Then compare with a pass plus reservations.