4 nights in January

Switzerland’s best asset is the Lauterbrunnen Valley and winter may not be the best time to go into the mountains. A train from the Venezia S. Lucia station in Venice takes 7h 45m and has three transfers. You would waste a day getting there.
A train from Venezia S. Lucia to Bern takes 7h 15m with one transfer and you could use this as your base to explore the valley since traveling to the Lauterbrunnen is 1h 30m from Bern and also requires a transfer.
Another option is to take a train from Venezia S. Lucia to Luzern (6h 30m) and transfer in Milan and is probably your best bet. I’ve never been to Luzern but I’ve never heard of anyone not liking it. From Luzern you can take a direct train to Zurich Flughafen’s airport rail station (1h 15m) and fly home from Zurich.

What about 4 more days in Italy? Florence is a 4 day stop, and good place to fly home from. Great for art. Also wine country. Ravenna just S of Venice has wonderful history and mosaics.

Have you considered a mountain destination in Italy like Bolzano (3.5 hours train) or Cortina (2.0 hours train)?

I assume that you are not a usual European traveler. Thus, you need to understand a couple of points:

1) Tourist destinations are mostly open during tourist season, which is April-October.
2) In tourist towns, most restaurants and hotels will not be open.
3) Winter destinations (ski resorts) are more open, of course.

So, if you are going to Switzerland to "explore", things may not work out. If you are going to ski, things will be better.

Since it's your honeymoon, I'd blow some bucks and do the ski resort thing. Take lessons, hit the bunny hill. Maybe Switzerland, maybe Italy. Slovakia also has great skiing, and is probably a lot cheaper.

Thanks for the suggestion guys.
Honestly, Switzerland was not my first choice for this trip due to the extreme cold and things remaining closed but I had 4 days to spare and I heard similar things about italian destinations being closed due to off season.
I have received an itinerary from a local travel agent here, I would like to request your views on this.
Day 1 Arrive lucerne
Day 2 Engelberg by train and Mt. Titlis Rotair gondola, Ice Flyer, Evening cruise on Lake lucern
Day 3 Train to Interlaken and Glacier 3000
Day 4 Jungfrau mountain excursion by cog wheel train and stay at interlaken
Day 5 Train to Zurich

I am also open to suggestions to mountains in Italy for 4 days but we are not the museum and history kind, So I guess florence etc doesn't make sense.

Thanks in advance.

When we were in Lucerne in early November the lake cruises had closed for the season so I am not sure they would be operating in January.

From Venice it would be an easy option to head east to Verona and Lake Como then fly home from Milan. Verona is a gorgeous small city with a spectacular Roman forum, ruined Roman theatre and Juliet's balcony. Lake Como is stunning at any time of the year.

If you want mountains then the Dolomites are much closer than Switzerland and you could do two nights in Bolzano then on to Innsbruck for two nights and fly home from Munich.

I would not say that Switzerland has ‘extreme cold’, but I guess it depends on what you are used to. You can look up average daily temps in Jan and see what you think.

The winter season is quite a big thing here and you will find plenty of hotels and restaurants that are open.

Boats on the Lake of Thun run all year long. When I checked online it look like the same is true of the boats in Luzern. Timetable can be found here:

https://www.lakelucerne.ch/en/

The mountains look amazing in the winter, on a clear day they appear larger and closer than they really are. The outings on your proposed itinerary reflect that your travel agent knows this and wants you to experience the beauty of the season.

A unique experience could be a Fondue gondola

https://www.pilatus.ch/en/discover/activities/fondue-gondola

Interlaken area in winter

https://www.interlaken.ch/en/experiences/winter-wonderland/winter-activities

More ideas

https://swissfamilyfun.com/winter-switzerland-non-skiers/

Hope this helps you get a picture of what it is like here. Lots of luck in planning the remainder of your honeymoon and congratulations!

The Dolomites are a 3h train ride from Venice’s S. Lucia station to Bolzano with a connection in Verona.

I'd do something in Italy, but if you are interested in what the travel agent suggested, I would research all of the transit. Some things could be closed--certain trains and such were when we were there in October. I'd choose one place plus where ever you need to be the last night, not any other one-night stays.

Not a whole lot for non-skiers, but Zermatt and/or Saas-Fee (Last Christmas). You can enjoy the Stimmung and party times. Almost guaranteed snow. Pick a good hotel with wellness and half board. We liked here in Zermatt. Great food and the family went over and above: https://europe-zermatt.ch/

Thank you everyone for your valuable advice. I have been suggested Upper Engadin as an option but I feel its extremely cold for us.
We would like to have a central location and do day trips (not very far) to mountain tops with the swiss pass and experience snow on these trips. Would also love to visit a snow covered village to enjoy this.
Looking forward to help on a 5 night itinerary.

What is "extremely cold" for you? Sure, winter is cold in Switzerland, but it is warmer than in a large part of the US and Canada.
The Upper Engadin is a lovely idea, stunning region in the winter!
For a honeymoon, spending a couple of nights at one of the fancy hotels in Pontresina or in Sils Maria would be lovely. I would avoid St Moritz itself, it has less charm!