Lugano to Milano - 4 minutes to change trains, enough time?

If SBB says you can do it, then you should be able to do it. You will have to duck down the stairway from Platform 2 and walk a few feet for the stairway to Platform 1. It is a designed connection. There is also a ramp if you have rolling luggage. The RE-80 should already be in the station waiting for you. It is coming from Locarno and will arrive at XX:54, 4 minutes before your arriving train, so once everyone makes the change, it will depart.

But if you do miss it, just wait 30 minutes for the next train, although you will need a seat reservation for that one as it is an EC train. Or wait 60 minutes for the next RE-80.

Thanks, good to know. We have several transfers on that trip to Venice, so if we miss the Milano connection, we will miss the Veice connection.

When is your travel date. Going from Basel to Venice I assume.

If you are travelling on an Eurail Pass you have the option of just leaving Basel earlier.

The 4 minute transfer tells me that you are taking an combination of IC and RE80. None of these trains require reservations, so taking an earlier train is certainly an option.
You can also take EC services on that route, but for those you would need a reservation. The easiest place to get that is probably the Basel train station.

Have you booked anything yet? Because if it is Venice you are going to the easiest thing to do is probably take the direct Zurich - Venice train.

Otherwise: If you miss a connection due to a delay you take the next train. So if you arrive late in Milano and miss your connection to Venice you go to the Frecciadesk, (at the head of the platform) and have them exchange your ticket for one on the next departure. They should do that for free.