1. With 4 persons you can't book only 3 beds. Only children up to 5 can share a bed with someone else:
   https://www.nightjet.com/en/buchung/tarifinformationen/allgemein
   You could book a couchette for 4 persons though. The ÖBB website will book you in the same compartment and warn you if that is not possible anymore.
2. The new timetable starts on 11 December; booking is probably possible from mid October.
3. Compartments can be locked from the inside. I never had any problems on any night trains (used dozens all over Europe).

thank you!

so what happens if i book a 3 bed sleeper cabin for 4 persons (2 adults and 2 kids)?

i could get 1 bed in another cabin but i don’t use it?

You'll get the 4th bed in another compartment indeed. You could also make the 6-year old one year younger if you're sure he/she will fit in a bed together with someone else. For children up to 5 years, the ÖBB website will automatically assume they will share a bed.

You should look into the difference between (mostly 3 beds on top of each) sleeper (schlafwagen) and couchette (2x3 or 2x2 bunks) (liegewagen). In /en/ its all ´sleeper´ but thats a bit misleading.

The real sleepers tend to sell out pretty fast on NightJet. NightJet stock an vary a lot-a week or so ago I saw one passing by and this was rented/leased pretty old stock not from OeBB.

i read review that couchette are not as comfy as sleepers, u think?

Yes, sleepers are higher comfort than couchettes but I’ll bet that reviewer wasn’t having 4 people in a sleeper, probably solo or couple.

There is very little room in cabins and not much headroom in the beds when set for 3 bunks.

A couchette booked for exclusive use will allow you to have 2 bunks of 2.

Take a look at the nightjet website https://www.nightjet.com/en/ , this gives lots of details of the various options and decide for yourself.

Save your money and stay in Venice Mestre then travel through the alps in daytime.

2 choices via Vienna or via Verona. View are fantastic and you have a decent night's sleep and breakfast.

Also worth keeping in mind that very few sleeper cabins of any type are en-suite.

I suggest Venice Mestre rather than the old Venice because there are “real” mainstream hotels at a fraction of the price of the tourist hotels (B&B s really) on the islands. The old part is easily travelled to in about 15 - 20 mins for a few euros. Many of these hotels have family rooms etc at very reasonable prices.

All the trains leaving Venice St Lucia stop at Mestre station and there are several hotels in walking distance.